RFR: String deduplication for traversal GC

[Zhengyu Gu]

Please review the implementation of string deduplication for traversal GC.


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


Test:

   hotspot_gc_shenandoah (fastdebug + release)
   specJVM with -XX:+UseStringDeduplication (fastdebug)


Thanks,

-Zhengyu