RFR: Re-fix memory managers and memory pools usage and pause reporting

[Roman Kennke]

Seems reasonable.

We will/do have partial cycles though, and will likely have young/old
cycles in the future too. But we should take care of this when it
becomes reality.

Patch is good.
Thanks, Roman

> http://cr.openjdk.java.net/~shade/shenandoah/mx-pause-cycle/webrev.01/
> 
> We recently did the fix in MXBeans that made it report GC pauses only. Unfortunately, this wrecks up
> notifications about GC heap usage, because our complete cycle has concurrent phases that are
> responsible for reclaiming the memory. We cannot pass this information with GC-pause events only.
> 
> This change rewires our MXBeans handling: instead of "minor"/"major" managers that make little sense
> in Shenandoah context, we have two separate managers, one reporting statistics for the entire GC
> cycle ("Shenandoah Cycles"), and another is reports data specific to GC pauses ("Shenandoah
> Pauses"). New test validate these managers report sensible data.
> 
> This patch partially reverts the work Zhengyu did last week.
> 
> Testing: hotspot_gc_shenandoah, new tests, eyeballing MXBeans notifications
> 
> Thanks,
> -Aleksey
>