[keyserver] Test impact of skipping old sidebars in 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 D9335

Test Plan:
Run the script:

after filtering out old sidebars, Ashoat's ThreadStore is 28832851 bytes large

Corresponds to 27.50 MiB, which is a reduction of 60.16%.

Reviewers: atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D9336