chore: Speed up tx validation in sequencer

[spalladino]

The sequencer currently picks up every single transaction in the p2p layer, validates all of them, and then slices the one it strictly needs. We need to change the tx validator so it can be instructed to stop at a threshold (note that validating one by one without a shared context could lead to duplicate nullifiers). Also, we should remove from the sequencer the validations already performed by the p2p layer.