[alias-server] [p16] Update database on block

Summary:
T2922

Depends on D13152

Add new main() function that is the actual node app. The app will check for new aliases every time a block is found and add to the database.

MVP functionality less API endpoints.

Test Plan:
sudo systemctl mongod start
Run a Cashtab version capable of broadcasting alias txs
node index.js
Confirm alias functions and db write works correctly
Register some aliases at test address, confirm the app sees the txs
Wait for a block to be found
Confirm alias functions and db write works correctly

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D13162