RFR 8221507: Implement JFR Events for Shenandoah

[Roman Kennke]

> Please review this patch that adds support for two new JFR events
> ShenandoahHeapRegionStateChange and ShenandoahHeapRegionInformation.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8221507
> Webrev: http://cr.openjdk.java.net/~kdobson/53476/webrev/
> 
> The events appears to result in ~2% overhead though there is some
> uncertainty as to whether some of that is noise.
> 
> Thanks,
> 
> Ken Dobson

Very nice!

The overhead is only present with JFR active, I assume?

Have you run hotspot_gc_shenandoah tests?

Other than that, the patch looks good. Thank you!!

Roman