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

[William Kemper]

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

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Commit messages:
 - Use concurrent worker session for concurrent phase

Changes: https://git.openjdk.org/shenandoah/pull/548/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=548&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344670
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/548.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/548/head:pull/548

PR: https://git.openjdk.org/shenandoah/pull/548