[alias-server] Removing untested logic from parseWebsocketMessage

Summary:
T3060

Depends on D13566

In order to simplify future reviews, deprecate untested logic from parseWebsocketMessage. Replace with commented TODO statements. Add basic unit tests for the function.

Unused functions and mocks are removed from utils.js. Functions that remain and are not used by the app -- these will have unit tests added one at a time in future diffs, as the added app logic requires.

Test Plan: Review simplified logic in parseWebsocketMessage. npm test and all pass

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13568