RFR: 8256298: Shenandoah: Enable concurrent stack processing

[Zhengyu Gu]

Please review this patch that enables concurrent stack processing for Shenandoah GC.

After this patch, all root processing is done concurrently for concurrent GC.

Test:
- [x] hotspot_gc_shenandoah Linux x86_64 and x86_32
- [x] Nightly
- [x] tier1 with -XX:+UseShenandoahGC on Linux x86_32

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Commit messages:
 - Merge master
 - Merge branch 'JDK-8255765-isolate-gcs' into JDK-8256298-conc-stack-proc
 - Fixed indentation
 - More from Aleksey's review
 - cleanup
 - fix styles and cleanup
 - Removed unnecessary includes
 - Merge
 - Merge branch 'JDK-8255765-isolate-gcs' into JDK-8256298-conc-stack-proc
 - Merge branch 'master' into JDK-8255765-isolate-gcs
 - ... and 121 more: https://git.openjdk.java.net/jdk/compare/34eb8b34...4fd486b5

Changes: https://git.openjdk.java.net/jdk/pull/2185/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2185&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256298
  Stats: 649 lines in 19 files changed: 466 ins; 129 del; 54 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2185.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2185/head:pull/2185

PR: https://git.openjdk.java.net/jdk/pull/2185