- 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