[alias-server] Handle startup case with its own callback in parseWebsocketMessage

Summary:
T3060

parseWebsocketMessage is the main function that updates valid alias registrations. If the server has been shutdown, it is called immediately on app startup to bring the server to the latest chain state.

Otherwise it is always called on receiving a websocket message from chronik. Instead of shimming the special startup case into the BlockConnected callback, define its own callback.

Add TODOs for future functions and logic.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13592