do not log txs at info level

BEFORE:

```
./tm-bench -c 5 -r 1000 127.0.0.1:46657
Stats          Avg       StdDev     Max
Txs/sec        1826      843        2744
Blocks/sec     1.100     0.300      2
```

AFTER:

```
./tm-bench -T 30 -c 5 -r 1000 127.0.0.1:46657
Stats          Avg       StdDev     Max
Txs/sec        6120      1970       9776
Blocks/sec     1.000     0.000      1
```

mempool/mempool.go

@@ -255,7 +255,7 @@ func (mem *Mempool) resCbNormal(req *abci.Request, res *abci.Response) {
 				tx:      tx,
 			}
 			mem.txs.PushBack(memTx)
-			mem.logger.Info("Added good transaction", "tx", tx, "res", r)
+			mem.logger.Debug("Added good transaction", "tx", tx, "res", r)
 			mem.notifyTxsAvailable()
 		} else {
 			// ignore bad transaction