RFR: Refactoring GC phase and heap allocation tracking out of policy

[Zhengyu Gu]

Refactoring GC phase timing tracking and heap allocation tracking out of 
ShenandoahCollectorPolicy.


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


Test:

   hotspot_gc_shenandoah (fastdebug and release)

   Manually verified -Xlog:gc+stats output.

Thanks,

-Zhenngyu