RFR: Improve handling of cancellation after final mark

[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.

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Commit messages:
 - Improve handling of cancellation after final mark

Changes: https://git.openjdk.java.net/shenandoah/pull/109/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=109&range=00
  Stats: 9 lines in 5 files changed: 3 ins; 4 del; 2 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