[keyserver] Move threadStoreValidator to redux-state-responders

Summary:
This diff is a follow-up diff to D10294.

Initially when I tried updating threadStoreValidator I had some issues with Jest... and as before it was a circular dependency issue.

Since threadStoreValidator is only being used by initialReduxStateValidator, I moved it right beside (above) it.

Depends on D10294

Test Plan: CI/flow/etc. Will become clear whether or not this validator works when we get to web.

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10295