ceph: drop src address(es) from message header [new protocol feature]

The CEPH_FEATURE_NOSRCADDR protocol feature avoids putting the full source
address in each message header (twice).  This patch switches the client to
the new scheme, and _requires_ this feature on the server.  The server
will support both the old and new schemes.  That means an old client will
work with a new server, but a new client will not work with an old server.

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