[Shenandoah 11u] RFR: Root processing related shenandoah-11u backports

[Roman Kennke]

Hi Zhengyu,

Looks ok to me.

Thank you!
Roman


> Backport following CRs to Shenandoah 11u, also fixed an issue in early
> backport.
> 
> 8227101: Shenandoah: Use ShenandoahSharedFlag for claimed flag in
>          ShenandoahSerialRoot
> 
> 8227866: Shenandoah: Split weak root processing and class unloading in
>          parallel cleaning task
> 
> 8236902: Shenandoah: Missing string dedup roots in all root scanner
> 
> 8231999: Shenandoah: Traversal failed compiler/jsr292
>         /CallSiteDepContextTest.java
>   [partial]
> 
> 
> 
> Webrev:
> http://cr.openjdk.java.net/~zgu/shenandoah/sh_11u_root_proc_backports/webrev.00/
> 
> 
> 
> Test:
>   hotspot_gc_shenandoah (fastdebug and release)
> 
> Thanks,
> 
> -Zhengyu
>