[services] Backup - Add server reactor implementations - pull backup reactor

Summary:
Depends on D3466

Add implementation for the pull backup reactor

Test Plan:

cd services
yarn run-backup-service

Reviewers: geekbrother, palys-swm, varun, jimpo

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3517