Add checks to TxSelector & other:

- 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`

db/kvdb/kvdb_test.go

@@ -38,7 +38,7 @@ func TestCheckpoints(t *testing.T) {
 	defer assert.NoError(t, os.RemoveAll(dir))
 
 	db, err := NewKVDB(dir, 128)
-	assert.NoError(t, err)
+	require.NoError(t, err)
 
 	// add test key-values
 	for i := 0; i < 10; i++ {