mds/MDSRank: improve is_stale_message to handle addrvecs

mds/MDSRank: improve is_stale_message to handle addrvecs

If we get a connection on a loopback from ourselves, get_source_addrs()
will have everything we bound to, but the mdsmap may only have the v1
address.  Avoid the addrvec comparison by instead comparing the
ConnectionRefs.

NOTE: this implementation is a stopgap.  We should really maintain a map
of ConnectionRefs for the current up set and compare the ConnectionRefs
directly instead of comparing addr(vecs).

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