RFR: 8344670: GenShen: Use concurrent worker session for concurrent mark phase

[William Kemper]

On Thu, 21 Nov 2024 00:31:24 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Concurrent/parallel worker sessions emit different JFR events. Mark is concurrent, so should emit `EventGCPhaseConcurrent`.

@shipilev caught it!

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PR Comment: https://git.openjdk.org/shenandoah/pull/548#issuecomment-2489839358