Match categories on the centroid instead of the geometry

Match categories on the centroid instead of the geometry

Category search used to run against the place_classtype tables, which
hold a centroid and index it. Moving to the categories column also
moved the searches onto placex.geometry, which changed what the filter
means and traded a small centroid index for the 22GB geometry index
over the whole of placex.

Put both searches back on the centroid, where the combined
centroid/categories index can serve them. This also reverts the
geometry prefilter in the near search: it was there to prune the
non-spatial categories index, and the combined index prunes on its
own.

Note that the near_radius branch of the POI search is left alone. It
queried placex.geometry before this change as well.