Remove "dateTouched" from ConpherenceParticipant

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