Some scratching changes.

* Move follower rate over-ride to late in RateControl so that is_scratching is
  properly updated and the PositionScratchController is updated. This unbreaks
  scratching of followers.

* Add notifyScratching method to SyncableListener for Syncables to report when
  they start and stop scratching.

* For non-explicit master mode, allow scratching any deck to affect all decks by
  temporarily making the scratching deck master.