RFR: Use allocation counter instead of timestamp to track region's age

[Zhengyu Gu]

Use allocation counter to track heap regions' age for generational and 
LRU GC.

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

Test:

hotspot_gc_shenandoah: fastdebug and release


Thanks,

-Zhengyu