Shenandoah: transit regions to cset state when adding to collection set

Current code has awkward ShenandoahCollectionSet method:

  // Bring per-region statuses to consistency with this collection.
  // TODO: This is a transitional interface that bridges the gap between
  // region statuses and this collection. Should go away after we merge them.
  void update_region_status();

...that is called after cset selection. It walks the regions again and transits the regions to cset status. At this point, we can just do this during SHCSet::add_region() and save a walk.