The SSDP library will log a warning on shutdown, as it doesn't expect the
UDP socket to be closed. Copy the hack on the Server implementation, which
avoids this warning getting logged, to the Client.
Due to the way SSDP works, Flamenco Manager needs to know its own URL,
where the Workers can reach it. These URLs are now found, and since there
can be multiple (like IPv6 + IPv4) they are all sent in a SSDP
notification as ;-separated strings.
