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

[Aleksey Shipilev]

On Tue, 25 Apr 2023 16:28:48 GMT, William Kemper <wkemper at openjdk.org> wrote:

> 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.

Marked as reviewed by shade (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/266#pullrequestreview-1400570519