Remove pointless ivar.
Don't enable infinite scroll on an empty thread list.
Record changes in AwfulFetchedResultsControllerDataSource for later playback.

Instead of always reloading the table view when unpaused, we can replay the
changes we did observe. This also fixes two bugs:

- Inserted sections have their rows inserted twice.
- Updated rows can trigger the wrong cell being "updated" with the wrong
  content.