added the closing operation of socket when a FaceRest instance is stopped.

soup_server_quit(), a function to stop soup server, doesn't close
the socket. After calling it, the creation of a new FaceRest
instance with the same port fails.

soup_server_disconnect() does not only have the effect of soup_server_quit()
but also close the socket.
https://developer.gnome.org/libsoup/stable/SoupServer.html#soup-server-disconnect