RemoveDuplicateAreaVisitor still pretty slow on some datasets

[bwitham]

just ran for ~43 minutes on a ~5.8 million feature dataset

need to try and speed it up some more

Unfortunately, can't remember what this data was... so find your own to test with :-( There are some pretty large datasets available in the regression test data.

[bwitham]

I'm not seeing any obvious ways in which this can be sped up. Made a small correction to the call to AreaCriterion. Will leave open for now in case any other ideas come into play.