[keyserver] Test impact of introducing ThreadRolePermissionsStore 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 D9338

Test Plan:
Run the script:

after introducing ThreadRolePermissionsStore, Ashoat's ThreadStore is 5640441 bytes large

Corresponds to 5.38 MiB, which is a reduction of 92.21%.

Reviewers: atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D9339