Add PoolL2Tx.Info about the status of the tx

PoolL2Tx.Info contains information about the status & State of the
transaction. As for example, if the Tx has not been selected in the last
batch due not enough Balance at the Sender account, this reason would
appear at this parameter.
This will help the client (wallet, batchexplorer, etc) to reason why a
L2Tx is not selected in the forged batches.

db/l2db/views.go

@@ -49,6 +49,7 @@ type PoolTxAPI struct {
 	Fee                  common.FeeSelector    `meddler:"fee"`
 	Nonce                common.Nonce          `meddler:"nonce"`
 	State                common.PoolL2TxState  `meddler:"state"`
+	Info                 *string               `meddler:"info"`
 	Signature            babyjub.SignatureComp `meddler:"signature"`
 	RqFromIdx            *apitypes.HezIdx      `meddler:"rq_from_idx"`
 	RqToIdx              *apitypes.HezIdx      `meddler:"rq_to_idx"`
@@ -90,6 +91,7 @@ func (tx PoolTxAPI) MarshalJSON() ([]byte, error) {
 		"fee":                         tx.Fee,
 		"nonce":                       tx.Nonce,
 		"state":                       tx.State,
+		"info":                        tx.Info,
 		"signature":                   tx.Signature,
 		"timestamp":                   tx.Timestamp,
 		"batchNum":                    tx.BatchNum,