[keyserver] Filter fetchThreadInfos queries to only look at viewer's threads

Summary:
This attempts strategy 3 outlined in ENG-3877.

Depends on D8513

Test Plan:
I started this task by testing various queries in the production environment to check performance.

After implementing the change, I used this patch to check performance on the JS side in my dev environment, and to output raw queries so I could test their performance in production: https://gist.github.com/Ashoat/220bb35da34ac7820867ac874d87beab

I found that for all users other than me, this change significantly sped up fetchThreadInfos.

I then deployed this to production to test performance more reliably. Hopefully so far no issues!

Reviewers: tomek, atul

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D8514