[services] Backup - pull backup reactor - initialize get reactor

Summary:
Depends on D3632

Adding basic structure for PullBackupReactor as well as the logic for initializeGetReactor.

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/D3633