Optimize filtering private threads when querying Conpherence

Summary:
Because most threads are private, this query can overheat the policy filter (today, probably only on this install).

Improve the common case by skipping "Visible To: Room Participants" threads if the viewer isn't a participant. This means they don't hit the application and don't count toward overheating the filter.

Test Plan: Viewed Conpherence threads.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D16740