ZGC: Expose GarbageCollectorMXBeans for both pauses and cycles

JDK-8240679 introduced a change where the information exposed via the GarbageCollectorMXBean went from being related to the GC pauses to instead be related to the GC cycles. This helped provide more accurate heap usage information. However, some users have noticed that that you no longer get timing information related to the GC pauses, only related to the GC cycle.

Shenandoah has opted for having two distinct memory managers to provide timing information about both pauses and cycles. To align how concurent GCs report this information, ZGC should probably do the same.