RFR: More cancelled concgc check and bailout

[Roman Kennke]

> More cancelled concgc check and bailout. With this patch, traversal GC 
> passed specJVM  with string deduplication on.
> 
> 
> Webrev: 
> http://cr.openjdk.java.net/~zgu/shenandoah/traversal_cancelled_gc/webrev.00/ 
> 
> 
> 
> Test:
>    hotspot_gc_shenandoah (fastdebug + release)
> 
> Thanks,
> 
> -Zhengyu
> 

Looks good to me. Thanks!