Reference class handling overhaul.

Reference class handling overhaul.

* Do not mark through soft-reachable referents when scanning f-
  reachable objects.

* Make the SoftReference clearing policy local to better conform to
  developer expecataions.

* Eliminate SCHEDULED_REFERENCE_MAGIC, a hold-over from a previous
  reference management algorithm.

* Move soft, weak and phantom reference processing into their own
  subroutines.

Change-Id: I992788d58f10fa08336e6ac3f0a4dbfa3fc2086f