Update TxSelector to return CoordIdxs used & other

- 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

db/kvdb/kvdb.go

@@ -123,7 +123,7 @@ func (kvdb *KVDB) reset(batchNum common.BatchNum, closeCurrent bool) error {
 		}
 		kvdb.db = sto
 		kvdb.CurrentIdx = 255
-		kvdb.CurrentBatch = batchNum
+		kvdb.CurrentBatch = 0
 
 		return nil
 	}
@@ -210,6 +210,11 @@ func (kvdb *KVDB) ResetFromSynchronizer(batchNum common.BatchNum, synchronizerKV
 	if err != nil {
 		return tracerr.Wrap(err)
 	}
+	// get currentIdx
+	kvdb.CurrentIdx, err = kvdb.GetCurrentIdx()
+	if err != nil {
+		return tracerr.Wrap(err)
+	}
 
 	return nil
 }