Free confdb message holder list on confdb exit

This operation will not only properly free memory, but also decreases
connection reference(s). This should solve deadloop in
coroipcs_ipc_service_exit, because confdb_exit_fn removes not only
notify_pipe but also deletes poll_dispatch, so nobody is left to
unreference connections.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Fabio M. Di Nitto <fdinitto@redhat.com>