Document how to build node

Replace github.com/marusama/semaphore by golang.org/x/sync

Feature/float40upgrade

- Add AmountF new parameter to ZKInputs
- Update ZKInputs test vectors with float40 checked with circom circuits
- Small fix at eth/rollup.go with lenL1L2TxsBytes with new length of
Float40

Migrate all packages to use Float40 & Add more test vectors at common

This commit adds Float40 related methods, and keeps the Float16 version
which will be deleted in a near future once the Float40 migration is
ready.

Fix txselector TransferToBJJ behaviour

Add semaphore for API queries to SQL

Fix l2db reorg of forging l2txs

Fix synchronizer, add verifier index config param

- eth
    - In EventsByBlock calls ignore blockNum if blockHash != nil.  This fixes
      the issue where a blockNumber and blockHash was being passed, which the
      eth events query function doesn't allow, causing the synchronizer to fail
      at every iteration.
- Node/Config
    - Add Coordinator.Debug.RollupVerifierIndex to force choosing a particular
      verifier by index in the Rollup smart contract.

Pass StateDB constructor parameters as Config type

- KVDB/StateDB
        - Pass config parameters in a Config type instead of using many
          arguments in constructor.
	- Add new parameter `NoLast` which disables having an opened DB with a
	  checkpoint to the last batchNum for thread-safe reads.  Last will be
	  disabled in the StateDB used by the TxSelector and BatchBuilder.
	- Add new parameter `NoGapsCheck` which skips checking gaps in the list
	  of checkpoints and returning errors if there are gaps.  Gaps check
	  will be disabled in the StateDB used by the TxSelector and
	  BatchBuilder, because we expect to have gaps when there are multiple
	  coordinators forging (slots not forged by our coordinator will leave
	  gaps).

Fix eth events query and sync inconsistent state

- kvdb
	- Fix path in Last when doing `setNew`
	- Only close if db != nil, and after closing, always set db to nil
		- This will avoid a panic in the case where the db is closed but
		  there's an error soon after, and a future call tries to close
		  again.  This is because pebble.Close() will panic if the db is
		  already closed.
	- Avoid calling pebble methods when a the Storage interface already
	  implements that method (like Close).
- statedb
	- In test, avoid calling KVDB method if the same method is available for
	  the StateDB (like MakeCheckpoint, CurrentBatch).
- eth
	- In *EventByBlock methods, take blockHash as input argument and use it
	  when querying the event logs.  Previously the blockHash was only taken
	  from the logs results *only if* there was any log.  This caused the
	  following issue: if there was no logs, it was not possible to know if
	  the result was from the expected block or an uncle block!  By querying
	  logs by blockHash we make sure that even if there are no logs, they
	  are from the right block.
	  	- Note that now the function can either be called with a
		  blockNum or blockHash, but not both at the same time.
- sync
	- If there's an error during call to Sync call resetState, which
	  internally resets the stateDB to avoid stale checkpoints (and a
	  corresponding invalid increase in the StateDB batchNum).
	- During a Sync, after very batch processed, make sure that the StateDB
	  currentBatch corresponds to the batchNum in the smart contract
	  log/event.

Update ZKInputs to incl Accumulated Fees of val 0

Fixes the case where there is a PoolTxs of type Exit with fee 0 for a
TokenID. Before this commit the Coordinator was not sending the
accumulated fee (which has value 0) to the Coordinator account for that
TokenID, with this commit the Coordinator always sends the accumulated
fee even when the value is 0 to match the hermez circom circuits
behaviour.

Also added a test to check the values and that also sends the proof to a
real proof server to check that can generate a valid proof.

Add Last db view in kvdb and statedb

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.

Add PoolL2Tx.Info about the status of the tx

PoolL2Tx.Info contains information about the status & State of the
transaction. As for example, if the Tx has not been selected in the last
batch due not enough Balance at the Sender account, this reason would
appear at this parameter.
This will help the client (wallet, batchexplorer, etc) to reason why a
L2Tx is not selected in the forged batches.

Swap proof b vector

Improve stateDB reliability

- 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.

Update TxID to use eth Keccak256

Fix api resopnse format for recommended fee

- Update TxID to use eth Keccak256
- Added more tests to L2Tx TxID calculation to check compatibility with
js version from https://github.com/hermeznetwork/hermezjs/pull/57

Update README

Use Warn instead of Error on get prices error

Update contracts and get verifiersLen from SC

Fix incorrect initial CurrentIdx in kvdb

Check enough funds on sender at TxSelector and don't include the tx in the selection

- TxSelector
	- Add check enough funds on sender at TxSelector and don't
include the tx in the selection
- TxProcessor
	- Add checks that the balance when substracted the
amount/amount+fee never goes below 0

fix #502

Feature/txid fixcollisions

L2Tx.TokenID is not on the data obtained by the Synchronizer from the
blockchain, but is set by the TxProcessor when processing the
transactions in the StateDB.

run integration on push to master