[services] Backup/Blob - Pull backup logic fixes

Summary:
Depends on D3729

While testing I spotted a few problems in the logic for pulling the backup. I'm going to describe each one of them in the inline comments.

Test Plan: Same as in D3535

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3738