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

[Poonam Bajaj]

These changes backport the following fixes to jdk8u-ri:
- 8071507: (ref) Clear phantom reference as soft and weak references do
- 8071931: Return of the phantom menace
- 8143847: Remove REF_CLEANER reference category

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

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

Changes: https://git.openjdk.java.net/jdk8u-ri/pull/4/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk8u-ri&pr=4&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8071507
  Stats: 170 lines in 10 files changed: 105 ins; 48 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk8u-ri/pull/4.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk8u-ri pull/4/head:pull/4

PR: https://git.openjdk.java.net/jdk8u-ri/pull/4