[alias-server] Refactor to support unit testing of API endpoints

Summary:
T3060

Refactor to support unit testing of API endpoints.

Following express documentation and mochajs example at https://github.com/mochajs/mocha-examples/tree/master/packages/express-rest-api

Remove the express app from main.js. Refactor index.js so that the app initializes the db first before passing this to the express server and the indexer.

Add unit test for existing API endpoint.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14113