[keyserver] Split fetchServerThreadInfos into two queries

Summary: Addresses ENG-3606.

Test Plan:
I tested by comparing the JSON.stringify'd results before and after this change using this patch: https://gist.github.com/Ashoat/f07fc27a8fe4c456fd5d675a802b3cad

No differences were found.

I'm now in the process of deploying this change to prod so we can test it there.

Reviewers: tomek, atul, jon

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D7315