[alias-server] Implement mock mongo db and test it

Summary:
T3060

Depends on D13634

Get initial config correct for using an in-memory mongodb instance for later unit tests.

Note: Future diffs will introduce more sophisticated tests, even for initializeDb, this is a stub.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13644