RFR: Fix ShenandoahPurgeSATBTask [v3]

[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 ...

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Removed unused _claim_token

-------------

Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/23/files
  - new: https://git.openjdk.java.net/shenandoah/pull/23/files/b9c27ee8..12e380b7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=23&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=23&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 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