empty array instead of 404

Update ExitProofs-Siblings padding

Load an ethereum keystore when the node is started in coordinator mode.  The
private key corresponding to the forger address must be imported into the
keystore before running the node in coordinator mode.  You can see an examples
in `cli/node/load-sk-example.sh`.

Include addrs in exit responses

Fix next forgers when no bids

Abstract KVDB from StateDB

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

Fix next forgers api response

Fix item_id order in forged l1UserTxs

Also, make sure that all SQL queries that return slices are sorted

Abstract TxProcessor from StateDB

- Abstract TxProcessor from StateDB
- Upgrade to last version of go-merkletree for the key-value DB usage

Fix ZKI NTx for MaxTx

AccountCreationAuth compatible with js implementation

On the Circuits uses the name `nTx` (and on the tests `maxTx`), but
Contracts `maxTx`. On Go we had both variables, but really only used
one, now at Go we use `MaxTx` to refer to the maximum number of
transactions.

Delete old checkpoints in stateDB automatically & Don't log errors when context done

Introduce a constructor parameter for the StateDB called `keep`, which tells
how many checkpoints to keep.  When doing a new checkpoint, if the number of
existing checkpoints exeeds `keep`, the oldest ones will be deleted.

TxSel return of AccCreatAuths from the selection

Update AccountCreationAuth & fix auth.HashToSign

- Fix AccountCreationAuth.HashToSign (was using `[]byte("0x...")`, which
uses the bytes of the string. Now uses the bytearray of the compressed
BJJ public key (compatible with js implementation))
- Update AccountCreationAuth to last specification (add to hash
parameters ChainID & HermezAddress)
- Add missing test to AccountCreationAuth

Add return of AccountCreationAuths at the TxSelector, which is an array
of bytearrays with the signatures of the AccountCreationAuthorization of
the accounts of the users created by the Coordinator with
L1CoordinatorTxs of those accounts that does not exist yet but there is
a transactions to them and the authorization of account creation exists.

Sync ForgerCommitment and use it in coord

Previously the coordinator was erroneously using Slot.BatchesLen to determine
when anyone can forge.  The correct behaviour is implmenented with the boolean
flag `ForgerCommitment`, that is set to true only when there's a batch before
the deadline within a slot.

Delete Slot.BatchesLen, and the synchronization code of this value from the
Synchronizer, as this is not needed

Parametrize ChainID

Fix StateDB & Til Nonce increment

Add forged txs in batch endpoints response

TxSelector update txs processing logic

Implement Pipeline.prepareForgeBatchArgs()

Add bootCoord to coord table, get coord by forgerAddr

- Add TestZKInputsEmpty circom test vectors
- Fix ZKInputsHash OldLastIdx bytearray

Fix API nonces

Update packr to 2.8.1 which fixes this issue:
https://github.com/gobuffalo/packr/issues/261 which I encountered while running
a test with `-race`

- Implement Pipeline.prepareForgeBatchArgs()
- Add a minimal stress test for the coordinator (that also runs the
  synchronizer)
- Update txprocessor.ProcessTxs() to return valid results for batches without
  transactions
  	- Add the boilerplate for the corresponding test, leaving as TODO the
	  zkInput values
- Update prover client to use the same point format as proof server (projective)
- Update interface of TxSelector.GetCoordIdxs to also return the authorizations
  to create accounts that go with the l1CoordinatorTxs.

Test purger, fix some nonces