ceph: discard incoming messages with bad seq #

We can get old message seq #'s after a tcp reconnect for stateful sessions
(i.e., the MDS).  If we get a higher seq #, that is an error, and we
shouldn't see any bad seq #'s for stateless (mon, osd) connections.

Signed-off-by: Sage Weil <sage@newdream.net>