RFR: Eliminating string dedup dependency on the second bitmap

[Zhengyu Gu]

By pre-evacuating String dedup table, and using read/write barriers to 
keep dedup table consistent, we can eliminate the dependency on 
completed bitmap during piggyback UR phase.

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

Test:
   hotspot_gc_shenandoah (fastdebug + release)

Thanks,

-Zhengyu