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.

- Close StateDB when stopping the node
- Lock the StateDB when doing checkpoints to avoid multiple instances of
  oppening the pebble DB at the same time.

- 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`

- Node
	- Load Coordinator Fee Account from config
		- Sign the AccountCreationMsg to generate the
		  AccountCreationAuth
		- Resolve #465
	- Wait for synchronizer termination before stopping coordinator to avoid
	  getting stuck when closing in the following case:
		- The coordinator stops reading the synchronizer msg channel,
		  and the node gets stuck sending a message to that channel.
- Common
	- Move account creation auth signature code to common.
	- Update RollupConstInputSHAConstantBytes
- Coordinator
	- Set batch status in the debug file
	- Propagate SCVariables on reorg
	- Pipeline: Get SCVariables updates
		- Resolve #457
	- Fix off by 1 error in Pipeline.shouldL1L2Batch() (which shouldn't have
	  caused any problem, but it was not right)
- KVDB
	- Delete future checkpoints after reset
	- In `ResetFromSynchronizer`, remove all checkpoints first, and follow
	  the same logic as `reset()`.
- Cli
	- Add command to generate a BabyJubJub key pair (to be used for the
	  Coordinator Fee Account)
- Node
	- Adjust example config `Coordinator.L1BatchTimeoutPerc` to avoid
	  missing the L1Batch deadline with the following setup:
		- a block is mined every 2 seconds
		- single proof server that takes 2 seconds to calculate a proof
- TxProcessor
	- Close temporary pebble used for the exit tree after usage.
		- Resolve #463

- Upgrade go-merkletree version to include the last changes of Pebble
that fixes the cgo issues (which should fix #453), from:
c2b05f12d7
- TxSelector
	- Remove parameter batchNum for GetL2TxSelection & GetL1L2TxSelection
	- Add checks of ToBJJ & ToEthAddr when ToIdx>255
	- Avoid getting the sender account twice to get the TokenID of a l2tx
	- Add test to check that selected L2Txs are sorted by Nonce
	- Discard L2Tx that return error at ProcessL2Txs
- executed `go mod tidy`

- StateDB
  - Update GetIdxByEthAddrBJJ to return ErrToIdxNotFound when idx not found, so can be checked at upper levels
- TxSelector
  - rm CoordIdxsDB that is no longer needed (also related methods)
  - add `getCoordIdx` method to get the Coordinator Idx for a given TokenID
  - Update coordinator account creation related to new TokenIDs from L2Txs
  - Reorganize GetL1L2TxSelection
  - return CoordIdxs used in the selection
- Update go-merkletree version which avoids marshaling Siblings to json
with 'null' value in case of empty array

- KVDB contains the Checkpoint & Resets system
- StateDB uses KVDB and adds all the StateDB related methods