messages_dgm: Properly handle receiver re-initialization

messages_dgm: Properly handle receiver re-initialization

This only properly covers the small-message nonblocking case. Covering
the large-message and the blocking case is a much larger effort assuming
we want to re-send the failed message if parts of the message has gone
through properly. Don't do that for now.

This was found by sanba_dnsupdate constantly recreating its irpc handle to
winbindd in the RODC case.

The messaging_dgm code cached connected datagram sockets based on the
destination pid for 1 second. Which means the IRPC responses from
winbindd are never delivered to samba_dnsupdate,
which will then hit a timeout.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13786

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