The beginnings of the LocalData tile server

@hampelm's building blocks of the tile server, with some housekeeping/refactoring from @prashtx, including:  
- 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

/cc @hampelm