[alias-server] Add new database methods for getting and storing aliases

Summary:
T3060

Add new methods for getting and storing aliases in db.

Note: Realized I need to wipe the db in between these unit tests. Next diff will fix this, improve organization of db unit tests.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13705