Consumer notification of disconnect

[fpagliughi]

Currently the consumer channel is not notified if the client loses connection to the broker. The client might be left hanging if a disconnect occurs.

let rx = cli.start_consuming();
for msg in rx.iter() {
    // This should break on a disconnect
}
// The client can reconnect here

[fpagliughi]

The message callback now passes an Option instead of just a message. None is sent if there's a problem with the client, such as it being disconnected. The channel is kept open, and after the problem is resolved, the application can just keep reading from it.