Use new smaller mapnik queryMany

This pull is dependent on: https://github.com/mapnik/node-mapnik/pull/301

Only remaining question: 

With the old form of `queryMany` we returned all features for a given vt. With the new form of queryMany, we only return the two closest features. However, with stacked polygons, the last feature, was the feature is feature on top and is not being returned with the new `queryMany`. @springmeyer, do you how mapnik chooses which features to go through first and if so, can it be reversed [here](https://github.com/mapnik/node-mapnik/blob/master/src/mapnik_vector_tile.cpp#L1067-L1122)?

cc @dnomadb