RFR: 8333109: GenShen: Factor generational mode out of gc helpers (redo) [v2]

[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.

William Kemper has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains two new commits since the last revision:

 - Restore cancellation check
 - Restore orginal isolate-gc-helpers commit
   
   This reverts commit 894aefc077d495b3cd892d339e9ae9120cf3be54.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/441/files
  - new: https://git.openjdk.org/shenandoah/pull/441/files/553192c6..13de0bc9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=441&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=441&range=00-01

  Stats: 144 lines in 5 files changed: 99 ins; 31 del; 14 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