RFR: Use state of mark context itself to track stability of old mark bitmap

[William Kemper]

Rely on the state of the bitmap itself, rather than the state of the GC for knowing when it is safe to use the old generation bitmap.

-------------

Commit messages:
 - Use state of mark context itself to track stability of old mark bitmap

Changes: https://git.openjdk.org/shenandoah/pull/266/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=266&range=00
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/266.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/266/head:pull/266

PR: https://git.openjdk.org/shenandoah/pull/266