RFR: 8263832: Shenandoah: Fixing parallel thread iteration in final mark task

[Zhengyu Gu]

Please review this trivial patch that parallelizes thread iteration in ShenandoahFinalMarkingTask

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - JDK-8263832-satb-remark

Changes: https://git.openjdk.java.net/jdk/pull/3080/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3080&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263832
  Stats: 12 lines in 1 file changed: 0 ins; 4 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3080.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3080/head:pull/3080

PR: https://git.openjdk.java.net/jdk/pull/3080