[keyserver] Introduce script to test ThreadStore reductions

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.

Test Plan:
Run the script:

before reductions, Ashoat's ThreadStore is 72374961 bytes large

Corresponds to 69.02 MiB.

Reviewers: atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D9335