Add a health check endpoint

[davenquinn]

In order to support Kubernetes liveness probes we need a stable way to evaluate service health. @brianaydemir is working on the infrastructure side of this.

The ideal approach would be /status or /healthcheck endpoint that returns 200 if the service is live. I'm not sure — should this hit the database? Perhaps it could return a count of cached tiles for different layers, if we wanted the endpoint to double as a "useful" view. But maybe it's better just to successfully return with no extra database work, especially if we intend this to be hit often.

[brianaydemir]

Testing connectivity to the database might be okay — even desirable, especially if the solution to "can't connect to the database" is "restart the Pod."

It sounds to me like everything except the response's status code is ignored, so there's value in computing more only if it lets us conclude that "restart the Pod" is necessary.

[davenquinn]

OK, I think to start we should just have something that responds 200 in all cases where a server connection is established. I think that restarting the Pod on a failed database connection is not the right answer, likely. So a basic "echo".

It'd be awesome if you could make the PR to implement this @brianaydemir