RFR: Clean up declarations and uses of marked_object_iterate

[Roman Kennke]

Ok

> http://cr.openjdk.java.net/~shade/shenandoah/cleanups-moi/webrev.01/
> 
> Continuing on cleaning up ShenandoahHeap interface. marked_object_iterate and friends can be sweeped
> up. This makes UR code a bit more straight-forward too.
> 
> Testing: tier3_gc_shenandoah (running)
> 
> Thanks,
> -Aleksey
>