[alias-server] [p13] Initialize mongodb

Summary:
T2922

Depends on D13117

Initialize a mongodb database in the app. This will be integrated later to store the most up-to-date valid aliases.

Test Plan:
Install and run mongodb locally (see instructions at https://www.mongodb.com/docs/manual/tutorial/install-mongodb-on-ubuntu/)
node db.js

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D13138