RFR: 8329350: GenShen: Do not reset mark bitmaps on a safepoint [v3]

[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).

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

  Improve comments and variable names per review feedback

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/413/files
  - new: https://git.openjdk.org/shenandoah/pull/413/files/1ef350b5..1d553587

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=413&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=413&range=01-02

  Stats: 14 lines in 3 files changed: 2 ins; 0 del; 12 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