Sets handler (fn [status]) for notification of channel being closed by the server or client. Handler will be invoked at most once. Useful for clean-up. Callback status argument: :server-close : Channel closed by sever :client-close : HTTP channel closed by client :normal : WebSocket closed by client (CLOSE_NORMAL) :going-away : WebSocket closed by client (CLOSE_GOING_AWAY) :protocol-error : WebSocket closed by client (CLOSE_PROTOCOL_ERROR) :unsupported : WebSocket closed by client (CLOSE_UNSUPPORTED) :unknown : WebSocket closed by client (unknown reason)