[services] Backup - Send log - Add basic logic

Summary:
Depends on D3535

Add a simple algorithm for handling receiving a new log. This is going to be extended in future diffs.

Test Plan:

cd services
yarn run-backup-service

Reviewers: palys-swm, geekbrother, varun, jimpo

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3558