[herald] function to handle block connected msg

Summary:
T2972

Depends on D13619

Move logic to handle BlockConnected msg to its own function. Add unit tests for failure and backup tg msgs.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13620