RFR: 8324067: GenShen: Isolate regulator thread to generational mode [v2]

[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.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 22 commits:

 - Merge branch 'shenandoah-master' into isolate-regulator-thread
 - Merge remote-tracking branch 'shenandoah/master' into isolate-regulator-thread
 - Clean up includes
 - Remove unnecessary mode checks
 - Remove non-generational functionality from regulator thread
 - Move heap changed tracking from control thread to heap
 - Clean up includes and headers
 - Factor allocation failure handling methods into common base class
 - Merge remote-tracking branch 'shenandoah/master' into isolate-regulator-thread
 - Merge remote-tracking branch 'shenandoah/master' into isolate-control-thread
 - ... and 12 more: https://git.openjdk.org/shenandoah/compare/8f4e6e22...6915cd07

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Changes: https://git.openjdk.org/shenandoah/pull/391/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=391&range=01
  Stats: 2311 lines in 17 files changed: 1449 ins; 731 del; 131 mod
  Patch: https://git.openjdk.org/shenandoah/pull/391.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/391/head:pull/391

PR: https://git.openjdk.org/shenandoah/pull/391