rebuild connectivity map

(2016-04-15T18:06:26Z) @kevinkreiser : originally created valhalla/baldr#243

in certain situations the set of tiles available to the routing engine may not be static. specifically if on a mobile device the user decides to download a new set of tiles or indeed they are downloading tiles on the fly, we'll want to either regenerate the connectivity map (geojson) or update the in memory data structure.