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

[Poonam Bajaj]

These changes backport JDK-8175797 and two very closely related fixes (8178832 and 8193780) to jdk8u-dev:

    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 these changes, the enqueue method clears the reference before enqueuing it to the registered queue.

It is a clean port from jdk8u-ri.

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

Changes: https://git.openjdk.org/jdk8u-dev/pull/99/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=99&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8175797
  Stats: 61 lines in 3 files changed: 52 ins; 3 del; 6 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/99.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/99/head:pull/99

PR: https://git.openjdk.org/jdk8u-dev/pull/99