RFR: 8052260: Reference.isEnqueued() spec does not match the long-standing behavior returning true iff it's in the ref queue

[Mandy Chung]

`Reference::isEnqueued` method was never implemented as it was initially specified since 1.2. The specification says that it tests if this reference object has been enqueued, but the long-standing behavior is to test if this reference object is in its associated queue, only reflecting the state at the time when this method is executed. The implementation doesn't do what the specification promised, which might cause serious bugs if unnoticed. For example, an application that relies on this method to release critical resources could cause serious performance issues, in particular when this method is misused on Reference objects without an associated queue.  `Reference::refersTo(null)` is the better recommended way to test if a Reference object has been cleared.

This proposes to deprecate `Reference::isEnqueued`.  Also the spec is updated to match the long-standing behavior.

CSR: https://bugs.openjdk.java.net/browse/JDK-8189386

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Commit messages:
 - add suppress warnings
 - 8052260: Reference.isEnqueued() spec does not match the long-standing behavior returning true iff it's in the ref queue

Changes: https://git.openjdk.java.net/jdk/pull/1684/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1684&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8052260
  Stats: 28 lines in 2 files changed: 22 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1684.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1684/head:pull/1684

PR: https://git.openjdk.java.net/jdk/pull/1684