[jdk8u-ri] RFR: 8175797: (ref) Reference::enqueue method should clear the reference object before enqueuing

[Poonam Bajaj]

These changes backport the following fixes to jdk8u-ri:

- 8175797: (ref) Reference::enqueue method should clear the reference object before enqueuing
- 8178832: (ref) jdk.lang.ref.disableClearBeforeEnqueue property is ignored
- 8193780: (ref) Remove the undocumented jdk.lang.ref.disableClearBeforeEnqueue system property

With this fix, the enqueue method clears the reference before enqueuing it to the registered queue.

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Commit messages:
 - 8175797: (ref) Reference::enqueue method should clear the reference object before enqueuing

Changes: https://git.openjdk.java.net/jdk8u-ri/pull/5/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk8u-ri&pr=5&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8175797
  Stats: 61 lines in 3 files changed: 52 ins; 3 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk8u-ri/pull/5.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk8u-ri pull/5/head:pull/5

PR: https://git.openjdk.java.net/jdk8u-ri/pull/5