RFR: Put regulator thread stop after gc thread iteration is prevented [v2]

[Kelvin Nilsen]

On Wed, 22 Mar 2023 22:37:16 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> Record that shutdown sequence has begun before we request the regulator thread to stop.
>
> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix numbering for comments

Marked as reviewed by kdnilsen (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/231#pullrequestreview-1354843660