RFR: 8259377: Shenandoah: Enhance weak reference processing timing tacking

[Zhengyu Gu]

Please review this enhancement for tracking weak references processing.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - cleanup
 - JDK-8259377: init update

Changes: https://git.openjdk.java.net/jdk/pull/1979/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1979&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259377
  Stats: 41 lines in 5 files changed: 12 ins; 2 del; 27 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1979.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1979/head:pull/1979

PR: https://git.openjdk.java.net/jdk/pull/1979