[keyserver] Remove holders in thread deleters

Summary:
Address ENG-9354.
Before calling DELETE FROM uploads, we need to select the extra column first and remove blob holders if present in the column JSON.

Depends on D13512

Test Plan: Enabled blob-hosted avatars for thin threads. Created a thread and set image avatar. Then deleted the thread. Looked into blob service logs to see that a blob was uploaded, then its holder was removed

Reviewers: tomek, ashoat

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D13513