Add Last db view in kvdb and statedb

Last db view is an opened pebble db which always contains a checkpoint from the
last batch.  Methods to access this last batch are thread safe so that views of
the last checkpoint can be made anywhere and with a consistent view of the
state.

api/txspool.go

@@ -170,7 +170,7 @@ func (a *API) verifyPoolL2TxWrite(txw *l2db.PoolL2TxWrite) error {
 		return tracerr.Wrap(err)
 	}
 	// Get public key
-	account, err := a.s.GetAccount(poolTx.FromIdx)
+	account, err := a.s.LastGetAccount(poolTx.FromIdx)
 	if err != nil {
 		return tracerr.Wrap(err)
 	}