The beginnings of the LocalData tile server

some housekeeping/refactoring

- use nodetiles-core just as a render stream for the PNGs, putting the routing and http response stuff in the server
- let us pipe the stream from nodetiles-core to the response, so we can later add png reduction/etc. into that pipe
- use Mongoose for the MongoDB connection, so we can take advantage of its connection pool and can use the high-level interface at our leisure
- use Express logging
- optionally use NodeFly and get the key from an environment variable

/cc @hampelm