s4/messaging: messaging_dgm_ref talloc hierarchy fix

s4/messaging: messaging_dgm_ref talloc hierarchy fix

Ensure the messaging dgm context goes away *before* the tevent
context. The messaging dgm context will likely have active fd or timer
events, their rundown will touch the associated tevent context.

Otoh, I deliberately don't free the imessaging context here, that's going
to happen as part of freeing the talloc_autofree_context() as before. I
think it suffers the same problem, eg imessaging_deregister() works on
an imessaging_context that might already be freed. But as it works,
don't change it.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>