msg/PipeConnection: make methods behave on 'anon' connection

The monitor does a create_anon_connection() to create a pseudo Connection
object for forwarded messages.  If we try to call mark_down or similar
on one of these we should silently ignore the operation, not crash.

If we try to send a message, still crash (explicitly assert); the caller
should probably know better.

Fixes: #9062
Signed-off-by: Sage Weil <sage@redhat.com>