RFR: 8324553: Shenandoah: Move periodic tasks closer to their collaborators

[William Kemper]

This is a refactoring to move Shenandoah's periodic tasks from the control thread to their respective dependencies: the pacer and monitoring support. This change will facilitate reviews of the upcoming changes to isolate the generational mode changes.

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Commit messages:
 - Add new line at end of file
 - Better names for things, remove unnecessary code
 - Move periodic tasks closer to their collaborators
 - Move periodic tasks into ShenandoahHeap

Changes: https://git.openjdk.org/jdk/pull/17540/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17540&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324553
  Stats: 214 lines in 8 files changed: 122 ins; 74 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/17540.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17540/head:pull/17540

PR: https://git.openjdk.org/jdk/pull/17540