Allow centralized sequencers to receive transactions directly

[S1nus]

roll-ups with centralized sequencers have no need to propagate transactions over p2p gossip since you know exactly where the transaction needs to go.

Sequencers can expose an RPC endpoint to receive transactions directly. Alternatively, it might be better to have them run a second server on a separate port to receive transactions directly.

• Create rpc method to receive transactions directly
• Send messages received by the RPC method into the node's incomingTxCh
• Process transactions received from the incomingTxCh

related to #675