Fix improper filtering behavior in ConpherenceParticipantQuery

Summary:
Pathway to D17685. This fixes an issue idenified in D17731: if any caller ever queried for more than one participant, some results could get thrown away by re-keying the results on thread PHID: two different participants can be members of the same thread!

This also fixes an issue from D17683, where a needParticipantCache() callsite was overlooked.

Test Plan:

• Viewed Conpherence dropdown.
• Sent messages, saw unread count / thread order still work properly.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17732