RFR: Refix ShenandoahPurgeSATBTask

[Zhengyu Gu]

My early fix was incomplete and incorrect.

1) ShenandoahPurgeSATBTask needs to change thread claim token for iterating threads in parallel, this was missing in early change.
2) Threads::threads_do() -> Threads::possibly_parallel_threads_do() was incorrect, the later version only iterates Java and VM threads. This part of changes are reverted.
3) A minor change: declaring _trashed_oops as volatile as it is a shared variable.

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Commit messages:
 - Refix ShenandoahPurgeSATBTask

Changes: https://git.openjdk.java.net/shenandoah/pull/25/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=25&range=00
  Stats: 12 lines in 1 file changed: 6 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/25.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/25/head:pull/25

PR: https://git.openjdk.java.net/shenandoah/pull/25