RFR: 8325516: Shenandoah: Move heap change tracking into ShenandoahHeap [v3]

[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.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Improve comment for heap_changed field

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17777/files
  - new: https://git.openjdk.org/jdk/pull/17777/files/e4b5ed74..9208bbba

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17777&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17777&range=01-02

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 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