[jdk8u-dev] RFR: 8071507: (ref) Clear phantom reference as soft and weak references do

[Poonam Bajaj]

Backport of 8071507 and 8143847 from jdk8u-ri to jdk8u-dev.

    8071507: (ref) Clear phantom reference as soft and weak references do
    8143847: Remove REF_CLEANER reference category

With these changes, phantom references are automatically cleared by the garbage collector like soft and weak references.

The changes are clean backport apart from a little manual adjustment for the hunk at line 246 in referenceProcessor.cpp.

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Commit messages:
 - 8071507: (ref) Clear phantom reference as soft and weak references do

Changes: https://git.openjdk.org/jdk8u-dev/pull/94/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=94&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8071507
  Stats: 170 lines in 10 files changed: 105 ins; 48 del; 17 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/94.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/94/head:pull/94

PR: https://git.openjdk.org/jdk8u-dev/pull/94