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

[Roman Kennke]

On Mon, 25 Jan 2021 15:18:02 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> 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

Looks good to me! Thank you!

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Marked as reviewed by rkennke (Reviewer).

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