RFR: Fix ShenandoahPurgeSATBTask [v2]

[Zhengyu Gu]

On Thu, 18 Mar 2021 20:15:52 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> Actually, now that I think about it a little more, I believe you should leave the claiming code where it was. It was one closure per task, and that should correctly claim the threads. Now you let all tasks see all the threads, which is wrong. Or I'm missing something.

The threads are claimed inside Threads::possibly_parallel_threads_do()

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PR: https://git.openjdk.java.net/shenandoah/pull/23