RFR (S) 8217343: Shenandoah control thread should be able to run at critical priority

[Roman Kennke]

Looks good to me. Thanks!

Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8217343
> 
> Fix:
>   http://cr.openjdk.java.net/~shade/8217343/webrev.01/
> 
> This allows for elevated thread priority for Shenandoah control thread. The change depends on
> JDK-8217378 fixed first.
> 
> Testing: hotspot_gc_shenandoah, new test
> 
> Thanks,
> -Aleksey
>