RFR 8221507: Implement JFR Events for Shenandoah

[Ken Dobson]

Hi all,

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