Post notifications asynchronously on main queue.

No need for observers to jump through hoops to get on the main queue.
Remove notification main queue contortions.

All of our notifications are always posted from the main queue.
Check for new tags up to thrice as often.
Simplify showing new thread tags.

Not sure why all that bookkeeping seemed necessary. Just reload the cells that
might have been affected and call it done.
Remove dead code.