[services] Backup - Add DB operations for Backup

Summary:
Database operations on BackupItems. We probably will not use remove method on production but it will be needed for tests.

There were some custom steps so I didn't fully use internal methods but after I implement operations for BackupItems and LogItems, I'm going to look at how we can take as much advantage of them as possible.

Depends on D3159

Test Plan: Backup service builds

Reviewers: palys-swm, geekbrother, varun, ashoat, jimpo

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3160