Conpherence - update the "unread" count in notifications panel aggressively

Summary:
Fixes T7561. Basically everytime we load some transactions in the thread manager, kick off an async thread to update the notification panel.

Should I consolidate this little bit of code into something like this._handleTransactionResponse(r)... ? I just want to keep the JS clear for other engineers and I wasn't sure if that was hiding a bit too much detail.

Test Plan: user a opened durable column. user b sent user a a few messages. reloaded user a page and noted the "N" count became N-1 as the message loaded. Switched messages and saw N-2, N-3, etc as I loaded up the messages.

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7561

Differential Revision: https://secure.phabricator.com/D12099