RFR: Improve handling of cancellation after final mark [v2]

[William Kemper]

> Handle case when gc is cancelled _after_ final mark but _before_ the control thread checks for cancellation. If final mark completes before the cancellation is detected, we can no longer degenerate to the marking phase.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Add clarifying comment

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/109/files
  - new: https://git.openjdk.java.net/shenandoah/pull/109/files/7bba584c..0a332758

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=109&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=109&range=00-01

  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/109.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/109/head:pull/109

PR: https://git.openjdk.java.net/shenandoah/pull/109