RFR: Fix ShenandoahPurgeSATBTask

[Zhengyu Gu]

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 ...

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Commit messages:
 - Fix ShenandoahPurgeSATBTask

Changes: https://git.openjdk.java.net/shenandoah/pull/23/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=23&range=00
  Stats: 17 lines in 1 file changed: 6 ins; 4 del; 7 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/23.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/23/head:pull/23

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