RFR: 8256298: Shenandoah: Enable concurrent stack processing [v4]

[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

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

  More of Roman's comments

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2185/files
  - new: https://git.openjdk.java.net/jdk/pull/2185/files/61b3dcb8..77582b33

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2185&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2185&range=02-03

  Stats: 8 lines in 2 files changed: 0 ins; 2 del; 6 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