[services] Backup - Database manager - Add findLogItem

Summary:
Depends on D4078

Adding findLogItem method to the backup's database manager.

Test Plan:
Services can still be built.

It's hard to really test this at this point. This method's going to be used later in the stack.

Reviewers: palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D4079