RFR (S) 8246100: Shenandoah: walk roots in more efficient order

[Zhengyu Gu]

Looks good in general. But not sure why call vm/weak/dedup roots limited 
parallel, I think they are fully parallel.

-Zhengyu

On 6/1/20 6:24 AM, Aleksey Shipilev wrote:
> RFE:
>    https://bugs.openjdk.java.net/browse/JDK-8246100
> 
> See the rationale in the RFE.
> 
> Webrev:
>    https://cr.openjdk.java.net/~shade/8246100/webrev.02/
> 
> Testing: hotspot_gc_shenandoah
>