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/statedb/utils_test.go

@@ -1,7 +1,6 @@
 package statedb
 
 import (
-	"fmt"
 	"io/ioutil"
 	"os"
 	"testing"
@@ -83,8 +82,7 @@ func TestGetIdx(t *testing.T) {
 	// expect error when trying to get Idx by addr2 & pk2
 	idxR, err = sdb.GetIdxByEthAddrBJJ(addr2, pk2.Compress(), tokenID0)
 	assert.NotNil(t, err)
-	expectedErr := fmt.Errorf("GetIdxByEthAddrBJJ: %s: ToEthAddr: %s, ToBJJ: %s, TokenID: %d", ErrToIdxNotFound, addr2.Hex(), pk2, tokenID0)
-	assert.Equal(t, expectedErr, tracerr.Unwrap(err))
+	assert.Equal(t, ErrIdxNotFound, tracerr.Unwrap(err))
 	assert.Equal(t, common.Idx(0), idxR)
 	// expect error when trying to get Idx by addr with not used TokenID
 	_, err = sdb.GetIdxByEthAddr(addr, tokenID1)