RFR: 8324067: GenShen: Isolate regulator thread to generational mode [v3]

[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 incrementally with one additional commit since the last revision:

  Remove unused code (design changed in upstream patch)

-------------

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/391/files
  - new: https://git.openjdk.org/shenandoah/pull/391/files/6915cd07..ce189e7b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=391&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=391&range=01-02

  Stats: 121 lines in 3 files changed: 0 ins; 121 del; 0 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