Display server status

[meguiraun]

Notify the user about the status of the server: 'BUSY', 'PROCESSING_REQUEST', 'DOWN'...

[johanfforsberg]

Since the server is asynchronous, there's currently not really a way to tell if it's "busy" (it's always possible to make more requests) but I think it would make sense to limit each client to one request at a time, since each new request means that we no longer care about the previous one anyway. This would require us to keep track of a client "session".

One way for the client to tell if the server is responding at all would be to do some form of "heartbeat" requests regularly, e.g. a HEAD request every ten seconds.