Add single connection support to Server

[KarstenB]

The src/asynchronous/server.rs currently assumes that there is a stream of incoming connections. For my scenario (NRI plugin) this is disadvantageous because I am already connected. Wrapping the connection into a Listener. with a once stream requires 'static.

There is also the downside that spawn_connection_handler hides potential errors:

    spawn(async move {
        conn.run()
            .await
            .map_err(|e| {
                trace!("connection run error. {}", e);
            })
            .ok();
    });

#292 proposes a solution by creating a start_connected method.