- call current_schema::mark_as_changed() directly
- call state_change::mark_as_changed() directly
- replaced SESSION_TRACKER_CHANGED with dummy tracker
- replaced Session_tracker::mark_as_changed() with
  State_tracker::mark_as_changed()
- hide and devirtualize original State_tracker::mark_as_changed(),
  rename it to set_changed()
- all implementations of mark_as_changed() now check is_enabled() for
  consistency
- no argument casts anymore