[services] Backup - Improve code style in send log reactor
Summary:
Depends on D3777
Add append holder logic to the add log reactor. In a case, the data already exists when adding a new log, we're just going to add a holder to the existing blob.
https://linear.app/comm/issue/ENG-672/create-addreferencetohashbyholder-in-the-blob-service
Test Plan: compile the service
Reviewers: palys-swm, ashoat
Reviewed By: palys-swm, ashoat
Subscribers: ashoat, palys-swm, Adrian, atul, benschac, yayabosh
Differential Revision: https://phabricator.ashoat.com/D3778