udp_recv: Close receiver after 5 seconds of inactivitly.

There is no way  for the UDP receiver to tell that para_server won't send any more
data, e.g. because the UDP target has been deleted. Currently,  in this case the UDP
receiver listens on its socket indefinitely and the buffer tree stays active as well. This
can be fatal if para_server starts sending data for another audio format much later.

So close the UDP socket if no data has been received for 5 seconds.