RFR: Fix ShenandoahPurgeSATBTask [v2]

[Roman Kennke]

On Thu, 18 Mar 2021 15:51:19 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> ShenandoahPurgeSATBTask looks wrong.
>> 
>> 1) Thread's token should be claimed by task, not by closure
>> 2) Threads::threads_do() is single thread version.
>> 
>> Note: It can be improved by only iterating JavaThread with ShenandoahJavaThreadsIterator, but it needs phase time ...
>
> Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove bad include

Ugh. Right. Looks good!

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Marked as reviewed by rkennke (Reviewer).

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