lib: Add messages_dgm_ref.[ch]

lib: Add messages_dgm_ref.[ch]

We only have one messaging_dgm context per process. But we will use this from
two completely independent messaging subsystems which are independently
initialized. We need to coordinate creation and destruction, do this via
talloc.

I know this looks like a step back, but when in the future we have really just
one messaging subsystem, this can go again. My immediate goal is to make
source3 and source4 transport-compatible, and this looks like a quick way
towards that goal.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>