RFR: Fix ShenandoahPurgeSATBTask [v2]

[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:

  Remove bad include

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

Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/23/files
  - new: https://git.openjdk.java.net/shenandoah/pull/23/files/7a0d62fd..b9c27ee8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=23&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=23&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 1 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