RFR: Use distinct "end of cycle" message for each Shenandoah pause [v2]

Tue May 2 23:24:51 UTC 2023

On Tue, 2 May 2023 23:20:06 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> This change allows the `GarbageCollectionNotificationInfo::gcAction` to be overridden. Shenandoah now uses this to distinguish the notification raised for each of the pauses during the concurrent cycle. This will make it easier for monitoring software to identify which phases may be having longer than expected pauses.
>
> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Move region aging into op_final_roots

Marked as reviewed by kdnilsen (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/270#pullrequestreview-1409998104