Switch to pulling tile index information from postgres server

This updates the low-level mapserver logic to pull layer information from the postgres server rather than an on-disk shapefile. This means that the "tile gen" component can completely stop creating/using shapefiles. However, these changes require a change to the database itself. The start_time column in the PG database needs to be of the timestamp type but is currently just a string. This requires updates to tile gen and to mapcache which write and read this time field. I have locally updated my tile gen repository to run ALTER TABLE on the database and make this change "in place". As long as the mapserver and mapcache deployments are updated at the same time we should see near-zero down time...I think.