Performance changes for staggered deadlines

This change improves the query when you have 35,000 open peer workflows and 0 assessments. Performance appears to still be good for a fluid workflow as well.

@wedaly @ormsbee 

FYI I had to add the "order by" clause back in when I filtered on graded_count. If I don't use graded_count, I don't have to use the order by clause. (I don't get that)