RFR: ShenandoahMemoryPool should support usage threshold

[Zhengyu Gu]

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