RFR: Move periodic GC decision making to base GC heuristics class

[Zhengyu Gu]

Please review this simple cleanup.

Currently, the same periodic GC decision is made by heuristics 
subclasses overall the places, while it can be made by base class.


Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/periodic_gc/webrev.00/

Test:

   tier3_gc_shenandoah (fastdebug and release)


Thanks,

-Zhengyu