rtsp-media: Add property to decide if sending media should be stopped when a client...

rtsp-media: Add property to decide if sending media should be stopped when a client disconnects without TEARDOWN

Without TEARDOWN it might be desireable to keep the media running and continue
sending data to the client, even if the RTSP connection itself is
disconnected.

Only do this for session medias that have only UDP transports. If there's at
least on TCP transport, it will stop working and cause problems when the
connection is disconnected.

https://bugzilla.gnome.org/show_bug.cgi?id=758999