gdbus: Fix race in name watching on connection teardown

If g_dbus_unwatch_name() is called from one thread at the same time as
the GDBusConnection is emitting ::disconnected in another thread, there
will be a race and the handler for ::disconnected may end up using
memory after it’s freed.

Fix this by serialising through the map_id_to_client, so that
on_connection_disconnected() atomically gets a strong reference to the
Client, or NULL.

https://bugzilla.gnome.org/show_bug.cgi?id=777307