Add PoolL2Tx verify signature needed methods

Implements the hash to be signed from PoolL2Tx, compatible with javascript version.
Implements the signature verification for a given PoolL2Tx.

db/l2db/l2db_test.go

@@ -149,7 +149,7 @@ func genTxs(n int) []*common.PoolL2Tx {
 			Fee:       99,
 			Nonce:     28,
 			State:     state,
-			Signature: *privK.SignPoseidon(big.NewInt(674238462)),
+			Signature: privK.SignPoseidon(big.NewInt(674238462)),
 			Timestamp: time.Now().UTC(),
 		}
 		if i%2 == 0 { // Optional parameters: rq