[keyserver] Test impact of improving encoding on Ashoat's ThreadStore

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 D9336

Test Plan:
Run the script:

after improving encoding, Ashoat's ThreadStore is 11123335 bytes large

Corresponds to 10.61 MiB, which is a reduction of 84.63%.

Reviewers: atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D9337