RFR: ShenandoahMemoryPool should support usage threshold

[Roman Kennke]

Ok!

Thanks,
Roman


> Webrev:
> http://cr.openjdk.java.net/~zgu/shenandoah/mpool_usage_threshold/webrev.00/
> 
> Support usage threshold to enable MBean
> java.management.memory.threshold.exceeded notification and to pass
> vmTestbase/nsk/monitoring/MemoryNotificationInfo/from/from001/TestDescription.java
> test, which will, otherwise, timed out.
> 
> The above test registers notification listener to MBean and expects to
> receive java.management.memory.threshold.exceeded notification. Without
> usage threshold support on VM side, will cause the test to wait forever,
> until timeout.
> 
> Test:
>   tier3_gc_shenandoah (fastdebug and release)
> 
> Thanks,
> 
> -Zhengyu
>