RFR: 8332081: GenShen: Disentangle generational mode update references

[William Kemper]

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

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Commit messages:
 - 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

Changes: https://git.openjdk.org/shenandoah/pull/430/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=430&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332081
  Stats: 438 lines in 6 files changed: 236 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