[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