Configurable fall-back

[zerebubuth]

At the moment, Cocotoca will 404 if it cannot find a z16 tile to cut up. Instead, it would be useful to be able to configure different fallback strategies:

1. Return 404. Most useful when used as a service behind a "dispatcher". At Mapzen we have used Fastly for this.
2. Request the z16 tile from upstream and cut the result. Most useful if you want to terminate z>16 processing at Cocotoca, and not handle these at any upstream point in the stack. For example, this would stop tileserver from putting z17 tiles into the TOI.
3. Proxy the whole request upstream, and return the result unchanged. Useful when the upstream service (like tileserver) is capable of handling any request at any zoom, and may cache the result (like tileserver).