Conpherence - fix recent participant cache

Summary: Ref T7795. This fixes the behavior where you end up with a "a, b, c..." as the list of participants, and yet user a just left.

Test Plan: joined and left a thread. verified database had correct values. observed correct behavior in messages dropdown

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7795

Differential Revision: https://secure.phabricator.com/D12338