RFR (S): Fix MXBean Full GC notifications

[Aleksey Shipilev]

Hi,

In JMH gc profiler, we have both "alloc" (actual allocations) and "churn" (space
freed by collections) counters. For Shenandoah, these counters disagree wildly,
because Shenandoah borks notifying MXBeans about Full GCs.

Fix:
 http://cr.openjdk.java.net/~shade/shenandoah/mx-fullgc-notify/webrev.01/

Thanks,
-Aleksey