RFR: 8325516: Shenandoah: Move heap change tracking into ShenandoahHeap

[William Kemper]

Shenandoah sets a flag when new regions are allocated or retired. This flag currently resides in the control thread. Moving it into the heap reduces code duplication with upcoming generational mode changes.

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Commit messages:
 - Move heap changed tracking into ShenandoahHeap

Changes: https://git.openjdk.org/jdk/pull/17777/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17777&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325516
  Stats: 24 lines in 4 files changed: 11 ins; 11 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17777.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17777/head:pull/17777

PR: https://git.openjdk.org/jdk/pull/17777