RFR: Fix deduplication assert and update comments

[Zhengyu Gu]

Aleksey fixed preclean task to use workers. Now, deduplication 
candidates should only come from workers.

Also, updated comments to reflect early changes, that we now identify 
candidates over all marking phases.

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


Test:
   tier3_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu