RFR: Fix deduplication assert and update comments

[Aleksey Shipilev]

On 10/03/2018 05:56 PM, Zhengyu Gu wrote:
> 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/

Looks good to me.

-Aleksey