Conpherence - fix disappearing older messages bug

Summary:
Fixes T8194. If you loaded a conpherence directly via a URI like /Z1 or /conpherence/1/ we didn't correctly cache the loaded transactions such that sending a message would only show that message and subsequent messages.

Fix is to build the cache properly in this case. Note this codepath is different because the server sends back more of the thread data in the initial request.

Test Plan: loaded a conpherence directly, sent messages, paged back, everything kept showing!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8194

Differential Revision: https://secure.phabricator.com/D12844