RFR: Only report GC pause time to GC MXBean

[Zhengyu Gu]

Only report GC pause time, instead of whole GC time span to GC MXBean, 
as CMS and G1 do.


Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/gc-mxbean/webrev.00/index.html


Thanks,

-Zhengyu