RFR: Shenandoah String Dedup refactoring

[Zhengyu Gu]

Hi,

This refactoring is based on upstream JDK-8203641, which is still under 
review 
(http://mail.openjdk.java.net/pipermail/hotspot-dev/2018-May/032565.html).

We initially borrowed G1's string deduplication implementation, but we 
diverged at one point, due to we may need to enqueue candidates inside 
write barrier. It is not the case anymore.

Once JDK-8203641 upstreamed, we can share dedup table and thread 
implementation, only need to plugin in our queue implementation.

It looks like StringDedupTable is another candidate to be replaced by 
ConcurrentHashTable, we should be able to benefit from it too.


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


Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu