RFR: 8332081: GenShen: Disentangle generational mode update references [v2]

[William Kemper]

> I believe the test failure on macosx is unrelated to these changes.

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

 - Merge remote-tracking branch 'shenandoah/master' into isolate-update-references
 - Remove single threaded usages of heap region iterator
 - Fix asserts to handle case when gc is cancelled
 - Move generational version of update refs to generational heap

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Changes: https://git.openjdk.org/shenandoah/pull/430/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=430&range=01
  Stats: 439 lines in 6 files changed: 237 ins; 185 del; 17 mod
  Patch: https://git.openjdk.org/shenandoah/pull/430.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/430/head:pull/430

PR: https://git.openjdk.org/shenandoah/pull/430