[services] Backup - Add find backup item to database manager

Summary:
Depends on D3629

We need a method in the database manager for finding a backup item by the backup id and user id.

Test Plan: The same as in D3535

Reviewers: palys-swm, geekbrother

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3630