RFR: 8329350: GenShen: Do not reset mark bitmaps on a safepoint

[William Kemper]

Shenandoah is currently resetting mark bitmaps during the init mark pause. This work should happen during the concurrent reset phase to avoid prolonging the safepoint. Also, free regions need to have the corresponding bitmap region reset or we risk having marked regions with no live data (which violates asserts during final mark).

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Commit messages:
 - Remove TODO comment (not going to do it here)
 - No thank you, CLION
 - Need to reset bitmaps for free regions
 - Do not clear mark bitmaps on a safepoint

Changes: https://git.openjdk.org/shenandoah/pull/413/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=413&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329350
  Stats: 134 lines in 11 files changed: 66 ins; 57 del; 11 mod
  Patch: https://git.openjdk.org/shenandoah/pull/413.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/413/head:pull/413

PR: https://git.openjdk.org/shenandoah/pull/413