New configuration Coordinator configuration parameter `ForgeOncePerSlotIfTxs`:
ForgeOncePerSlotIfTxs will make the coordinator forge at most one batch per
slot, only if there are included txs in that batch, or pending l1UserTxs in the
smart contract.  Setting this parameter overrides `ForgeDelay`,
`ForgeNoTxsDelay`, `MustForgeAtSlotDeadline` and `IgnoreSlotCommitment`.

Also restructure a bit the functions that check policies to decide whether or
not to forge a batch.

- Add new command to the cli/node: `serveapi` that alows serving the API just
  by connecting to the PostgreSQL database.  The mode flag should me passed in
  order to select whether we are connecting to a synchronizer database or a
  coordinator database.  If `coord` is chosen as mode, the coordinator
  endpoints can be activated in order to allow inserting l2txs and
  authorizations into the L2DB.

Summary of the implementation details
- New SQL table with 3 columns (plus `item_id` pk).  The table only contains a
  single row with `item_id` = 1.  Columns:
    - state: historydb.StateAPI in JSON.  This is the struct that is served via
      the `/state` API endpoint.  The node will periodically update this struct
      and store it int he DB.  The api server will query it from the DB to
      serve it.
    - config: historydb.NodeConfig in JSON.  This struct contains node
      configuration parameters that the API needs to be aware of.  It's updated
      once every time the node starts.
    - constants: historydb.Constants in JSON.  This struct contains all the
      hermez network constants gathered via the ethereum client by the node.
      It's written once every time the node starts.
- The HistoryDB contains methods to get and update each one of these columns
  individually.
- The HistoryDB contains all methods that query the DB and prepare objects that
  will appear in the StateAPI endpoint.
- The configuration used in for the `serveapi` cli/node command is defined in
  `config.APIServer`, and is a subset of `node.Config` in order to allow
  reusing the same configuration file of the node if desired.
- A new object is introduced in the api: `StateAPIUpdater`, which contains all
  the necessary information to update the StateAPI in the DB periodically by
  the node.

- Moved the types `SCConsts`, `SCVariables` and `SCVariablesPtr` from
  `syncrhonizer` to `common` for convenience.

Update to last Poseidon (go-iden3-crypto) & go-merkletree versions,
update the affected test vectors, and check ZKInputs compatibility with
last version of circuits.

Update to last Poseidon version (which includes the changes of the
reference implementation from
26ddaa91db)

Compatible with update at circomlib
(cf853c1cc9)

- cli / node
    - Update handler of SIGINT so that after 3 SIGINTs, the process terminates
      unconditionally
- coordinator
    - Store stats without pointer
    - In all functions that send a variable via channel, check for context done
      to avoid deadlock (due to no process reading from the channel, which has
      no queue) when the node is stopped.
    - Abstract `canForge` so that it can be used outside of the `Coordinator`
    - In `canForge` check the blockNumber in current and next slot.
    - Update tests due to smart contract changes in slot handling, and minimum
      bid defaults
    - TxManager
        - Add consts, vars and stats to allow evaluating `canForge`
        - Add `canForge` method (not used yet)
        - Store batch and nonces status (last success and last pending)
        - Track nonces internally instead of relying on the ethereum node (this
          is required to work with ganache when there are pending txs)
        - Handle the (common) case of the receipt not being found after the tx
          is sent.
        - Don't start the main loop until we get an initial messae fo the stats
          and vars (so that in the loop the stats and vars are set to
          synchronizer values)
        - When a tx fails, check and discard all the failed transactions before
          sending the message to stop the pipeline.  This will avoid sending
          consecutive messages of stop the pipeline when multiple txs are
          detected to be failed consecutively.  Also, future txs of the same
          pipeline after a discarded txs are discarded, and their nonces reused.
        - Robust handling of nonces:
            - If geth returns nonce is too low, increase it
            - If geth returns nonce too hight, decrease it
            - If geth returns underpriced, increase gas price
            - If geth returns replace underpriced, increase gas price
        - Add support for resending transactions after a timeout
        - Store `BatchInfos` in a queue
    - Pipeline
        - When an error is found, stop forging batches and send a message to the
          coordinator to stop the pipeline with information of the failed batch
          number so that in a restart, non-failed batches are not repated.
        - When doing a reset of the stateDB, if possible reset from the local
          checkpoint instead of resetting from the synchronizer.  This allows
          resetting from a batch that is valid but not yet sent / synced.
    - Every time a pipeline is started, assign it a number from a counter.  This
      allows the TxManager to ignore batches from stopped pipelines, via a
      message sent by the coordinator.
    - Avoid forging when we haven't reached the rollup genesis block number.
    - Add config parameter `StartSlotBlocksDelay`: StartSlotBlocksDelay is the
      number of blocks of delay to wait before starting the pipeline when we
      reach a slot in which we can forge.
    - When detecting a reorg, only reset the pipeline if the batch from which
      the pipeline started changed and wasn't sent by us.
    - Add config parameter `ScheduleBatchBlocksAheadCheck`:
      ScheduleBatchBlocksAheadCheck is the number of blocks ahead in which the
      forger address is checked to be allowed to forge (apart from checking the
      next block), used to decide when to stop scheduling new batches (by
      stopping the pipeline).  For example, if we are at block 10 and
      ScheduleBatchBlocksAheadCheck is 5, eventhough at block 11 we canForge,
      the pipeline will be stopped if we can't forge at block 15.  This value
      should be the expected number of blocks it takes between scheduling a
      batch and having it mined.
    - Add config parameter `SendBatchBlocksMarginCheck`:
      SendBatchBlocksMarginCheck is the number of margin blocks ahead in which
      the coordinator is also checked to be allowed to forge, apart from the
      next block; used to decide when to stop sending batches to the smart
      contract.  For example, if we are at block 10 and
      SendBatchBlocksMarginCheck is 5, eventhough at block 11 we canForge, the
      batch will be discarded if we can't forge at block 15.
    - Add config parameter `TxResendTimeout`: TxResendTimeout is the timeout
      after which a non-mined ethereum transaction will be resent (reusing the
      nonce) with a newly calculated gas price
    - Add config parameter `MaxGasPrice`: MaxGasPrice is the maximum gas price
      allowed for ethereum transactions
    - Add config parameter `NoReuseNonce`: NoReuseNonce disables reusing nonces
      of pending transactions for new replacement transactions.  This is useful
      for testing with Ganache.
    - Extend BatchInfo with more useful information for debugging

- eth / ethereum client
    - Add necessary methods to create the auth object for transactions manually
      so that we can set the nonce, gas price, gas limit, etc manually
    - Update `RollupForgeBatch` to take an auth object as input (so that the
      coordinator can set parameters manually)
- synchronizer
    - In stats, add `NextSlot`
    - In stats, store full last batch instead of just last batch number
    - Instead of calculating a nextSlot from scratch every time, update the
      current struct (only updating the forger info if we are Synced)
    - Afer every processed batch, check that the calculated StateDB MTRoot
      matches the StateRoot found in the forgeBatch event.

Last db view is an opened pebble db which always contains a checkpoint from the
last batch.  Methods to access this last batch are thread safe so that views of
the last checkpoint can be made anywhere and with a consistent view of the
state.

- cli / node
    - Update handler of SIGINT so that after 3 SIGINTs, the process terminates
      unconditionally
- coordinator
    - Store stats without pointer
    - In all functions that send a variable via channel, check for context done
      to avoid deadlock (due to no process reading from the channel, which has
      no queue) when the node is stopped.
    - Abstract `canForge` so that it can be used outside of the `Coordinator`
    - In `canForge` check the blockNumber in current and next slot.
    - Update tests due to smart contract changes in slot handling, and minimum
      bid defaults
    - TxManager
        - Add consts, vars and stats to allow evaluating `canForge`
        - Add `canForge` method (not used yet)
        - Store batch and nonces status (last success and last pending)
        - Track nonces internally instead of relying on the ethereum node (this
          is required to work with ganache when there are pending txs)
        - Handle the (common) case of the receipt not being found after the tx
          is sent.
        - Don't start the main loop until we get an initial messae fo the stats
          and vars (so that in the loop the stats and vars are set to
          synchronizer values)
- eth / ethereum client
    - Add necessary methods to create the auth object for transactions manually
      so that we can set the nonce, gas price, gas limit, etc manually
    - Update `RollupForgeBatch` to take an auth object as input (so that the
      coordinator can set parameters manually)
- synchronizer
    - In stats, add `NextSlot`