RFR: 8324067: GenShen: Isolate regulator thread to generational mode

[William Kemper]

Shenandoah's generational mode uses a second thread to evaluate heuristics. This is necessary so that the heuristics may interrupt a control thread which is running an old cycle in order to run a young cycle.

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Commit messages:
 - Merge remote-tracking branch 'shenandoah/master' into isolate-control-thread
 - Add missing precompiled header
 - Move common code from control threads to heap
 - WIP: Pulling things into common base class (a bridge too far?)
 - WIP: checkpoint
 - 8324553: Shenandoah: Move periodic tasks closer to their collaborators
 - 8324334: Shenandoah: Improve end of process report
 - Merge branch 'shenandoah-master' into isolate-control-thread
 - Minor cleanup
 - Put an interface between shenandoah heap and control threads
 - ... and 3 more: https://git.openjdk.org/shenandoah/compare/1ecdc046...2109a3d0

Changes: https://git.openjdk.org/shenandoah/pull/388/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=388&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324067
  Stats: 2373 lines in 16 files changed: 1574 ins; 674 del; 125 mod
  Patch: https://git.openjdk.org/shenandoah/pull/388.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/388/head:pull/388

PR: https://git.openjdk.org/shenandoah/pull/388