msg/async: allow anonymous client-side connections

msg/async: allow anonymous client-side connections

If the connection mode is lossy, allow us to open a new connection to
a target, regardless of whether other such connections are already open.
This allows for single-use connections.  If you call this multiple times,
you'll get separate, distinct connections.

We are lucky that the cleanup infrastructure for AsyncMessenger just works
without modification.  :)

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