gstrtspconnection: RTSP connection does not close after timeout

The watch object's "closed" function could be called after a timeout has occurred in either gst_rtsp_watch_write_data or gst_rtsp_source_dispatch_write. Right now this is not the case. For gst-rtsp-server, this results in an open thread after a client has timed out. This thread will normally not close until the TCP timeout of 15 minutes.