RFR: 8333109: GenShen: Factor generational mode out of gc helpers (redo)

[William Kemper]

This is the same as https://git.openjdk.org/shenandoah/pull/436, except that does _not_ remove a critical cancellation check. The comments around this cancellation check have been improved.

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Commit messages:
 - Merge remote-tracking branch 'shenandoah/master' into fix-isolate-gc-helpers
 - Improve comments and cancellation check
 - Improve comment regarding cancellation during mark
 - Restore wrong looking cancellation check

Changes: https://git.openjdk.org/shenandoah/pull/441/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=441&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333109
  Stats: 201 lines in 1 file changed: 33 ins; 109 del; 59 mod
  Patch: https://git.openjdk.org/shenandoah/pull/441.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/441/head:pull/441

PR: https://git.openjdk.org/shenandoah/pull/441