[lib] Fix up client ThreadStore validation

Summary:
I noticed we use tcomb validators on the client side here, so I figured it would be good to fix them before we insert any ThickRawThreadInfos into SQLite.

Depends on D12495

Test Plan: Flow

Reviewers: kamil, marcin

Reviewed By: marcin

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12496