8178832: (ref) jdk.lang.ref.disableClearBeforeEnqueue property is ignored
Summary: Delay property access to first use, and fix test.
Reviewed-by: mchung

8175797: (ref) Reference::enqueue method should clear referent before enqueuing
Reviewed-by: alanb, kbarrett, mr

8132306: java/lang/ref/ReferenceEnqueue.java fails with "RuntimeException: Error: poll() returned null; expected ref object"
Summary: Carefully order ref.queue and queue list updates to address races.
Reviewed-by: dholmes, dfuchs, plevart

4243978: (ref) Race condition in Reference.enqueue()
4268317: (ref) Reference.isEnqueued() can return true when instance not enqueued
Summary: The reference handler now declares, and assumes, that the discovered field, rather than the next field, is (to be) used to link the entries in the pending list, thus allowing a reference object to be safely enqueued even while it is in the pending state. Also added slightly modified regression tests from the two bug reports.
Reviewed-by: mchung, alanb, jcoomes