[services] Backup redesign - architecture

Summary:
This is an attempt to do an architecture review in phabricator as mentioned here.

We need to redesign the backup service, here is the task for it https://linear.app/comm/issue/ENG-205/redesign-backup-service.

https://linear.app/comm/issue/ENG-626/architecture-review

Very high-level diagram
https://www.figma.com/file/eraOvpUhVvLmoVwA9SfNWG/new-backup?node-id=0%3A1

Test Plan: -

Reviewers: palys-swm, geekbrother, ashoat, anunayk, jimpo, karol-bisztyga

Reviewed By: palys-swm, ashoat

Subscribers: anunayk, varun, benschac, ashoat, palys-swm, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D2950