R1:f46263903cbe

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 /…