RFR (M) 8232575: Shenandoah: asynchronous object/region pinning

[Roman Kennke]

Patch looks good! Thank you!

Roman


> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8232575
> 
> Current object/region pinning scheme bottlenecks on a lock, rendering some non-exceptional scenarios
> quite slow. The way out is to collect critical pins atomically, and then update the region states
> near the code that needs it (mostly selecting collection set). See the bug for more info.
> 
> Fix:
>   https://cr.openjdk.java.net/~shade/8232575/webrev.02/
> 
> Testing: hotspot_gc_shenandoah {fastdebug,release}; tier{1,2,3} with Shenandoah; GZIP workload with
> {normal, traversal} x {adaptive, aggressive}
>