[keyserver] Factor out logic for measuring / printing ThreadStore reduction results

Summary:
This diff stack introduces a script that tests various different approaches to reducing the size of the ThreadStore. We may or may not land it... the goal is just to get some numbers.

Depends on D9339

Test Plan: Run the script

Reviewers: atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D9340