Separate GeoJSON layers from tiles

Currently, shapes come from a geojson tile layer. This works well, but has several downsides, including refreshing the screen when zooming and a disconnect between logical layers (tract, county, metro) and ajax layers.

This uses an ajax layer for data and draws the received shapes into three geojson layers. As these layers don't get cleared, shapes remain on screen when zooming. This feels much more responsive.