Conpherence - fix unread counts

Summary: we weren't updating the "latest seen transaction PHID" properly. do that and ONLY do it from the view handler so we know the user got a real good chance of actually seeing the message. also we weren't searching through the transactions correctly; fix that.

Test Plan: sent a test user some messages. noted the proper count of unread messages.

Reviewers: epriestley, chad

Reviewed By: chad

CC: aran, Korvin

Maniphest Tasks: T2399

Differential Revision: https://secure.phabricator.com/D4677