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.

separated the policy to decide if we forge batch or not

Allow serving API only via new cli command

- 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.

Fix TxSel discard tx when ProcessL2Tx gives err

Refactor getL1L2TxSelection, which fixes some problems for certain
combinations of txs.

Add the option to forge batch at the slot deadline

Allow price update configuration to be specified per token

Update to last Poseidon & go-merkletree versions

Synchronize the AccountCreationAuths from L1CoordinatorTxs

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)

Add coingecko client to price updater

API doesnt accept pool txs without to

Fix NewFloat40Floor helper method

fix some code typos

Fix tx signature when toEthAddr starts with 0x0

Fix ZKInputs.ISExitRoot for case of L1CoordTxs

For the cases when there are Exit L1UserTxs and also there are
L1CoordinatorTxs, the ISExitRoot should have the expected value (!=0)
for the L1CoordinatorTxs slots.

Fix TxSel for unselected tx update internal Nonce

Fix TxSelector for unselected tx update internal Nonce.
Also updated TxSelector config parameter.

Add NewFloor40Floor helper method

Wipe StateDB in cli command wipesql

Also added tracerr.Wrap(...) to missing places.

Also, in the cli commands `wipesql` and `discard`, always rebuild the current
checkpoint of the stateDBs to make sure it's in a consistent non-corrupted
state and do a reset afterwards.  These commands will allow reverting the
StateDB to a valid and consistent state in case a crash leaves the StateDB in a
corrupted state.

Extend TxProcessor documentation and explanation

Update example config with working values

The `ForgeBatchGasCost` parameters work for the current version of the smart
contracts.

Add lll to linter checks

Updated:
	batchbuilder
	common
	coordinator
	db/statedb
	eth
	log
	node
	priceupdater
	prover
	synchronizer
	test/*
	txprocessor
	txselector

Pending (once
https://github.com/hermeznetwork/hermez-node/tree/feature/serveapicli is
merged to master):
	Update golangci-lint version to v1.37.1
	api
	apitypes
	cli
	config
	db/historydb
	db/l2db

Don't select extra columns in SQL, fix metrics query

In the metrics query, the estimatedTimeToForgeL1 was probably not being read
from the SQL query because it contained upper case letters, and in PostgreSQL
when identifiers are not double quoted they are case insensitive:
https://stackoverflow.com/a/20880247

When resending an eth tx, always increase gas price

Fix unforged L1UserTxs query in forgeBatch

Previously the code was only querying the unforged L1UserTxs of a particular
queue, but this was incorrect because there are always two non-forged queues:
the frozen one and the open one.

Replace it by a query to all the unforged L1UserTxs via a new HistoryDB method.

Incrementing the gas price unconditionally avoids sending the exact same
transaction, which geth will reject because that transaction already exists in
the pool

fix forge address balance printing removing the int64 casting

Avoid SQL error when checking value of token without usd on tx insert