RFR: Enable string deduplication in all marking phases

[Zhengyu Gu]

Aleksey found that Shenandoah is inconsistent on honoring string 
deduplication flag.

Shenandoah string deduplication was initially designed for concurrent 
marking phases only. Over the courses, it diverged. e.g. Traversal GC 
also performs deduplication in init and final pauses.

So, let's make it consistent:

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

Test:

   tier3_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu