RFR(S): ShenandoahCollectionSet refactor

[Zhengyu Gu]

Hi,

This refactor eliminates the second collection set state, which results 
out-of-sync problem we encountered early.

Now, ShenandoahCollectionSet wraps _cset_fast_test map, and all updates 
to the map have to go through this class.

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

Test:
   hotspot_gc_shenandoah with fastdebug and release

Thanks,

-Zhengyu