RFR(S) Queuing string dedup candidates when they are evacuating at safepoints

[Zhengyu Gu]

String dedup protocol requires deduplication outside of safepoints. When 
strings are evacuated during safepoints, we need to queue the candidates 
and process them by string dedup thread later.

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

Test:

hotspot_gc_shenandoah (fastdebug and release)


Thanks,

-Zhengyu