[services] Backup - Apply reading log chunk in send log reactor

Summary:
Depends on D3531

After reading the user id we want to read log chunks

Test Plan: backup service build

Reviewers: palys-swm, geekbrother, varun, jimpo

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3612