[alias-server] Make unconfirmed txs available to handleBlockConnected

Summary:
We will want to deal with unconfirmed txs on app startup (and block connected) to parse for pending alias registrations.

Update functions and unit tests to support this.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14467