Summary:
Pathway to D17685. This column is (mostly) a denormalization of `dateModified` on the thread.
Just use a JOIN instead.
This isn't //exactly// the same: we'll bump threads to the top now for non-message changes (e.g., a topic or title change). That seems fine, but we could put a `lastMessageDate` on Thread later if we want to refine it.
Also got rid of a lot of other unused stuff. There's a big garbage TODO here, I'll fix that in the next change.
Test Plan:
- Grepped for `dateTouched`.
- Grepped for `participantCursor`.
- Grepped for `ConpherenceParticipantQuery::LIMIT`.
- Looked for callsites to `setOrder()`, found none.
- Added a message to an older thread, saw it bump up to the top.
Reviewers: chad
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D17731
