libceph: ceph_connection_operations::reencode_message() method

Give upper layers a chance to reencode the message after the connection
is negotiated and ->peer_features is set.  OSD client will use this to
support both luminous and pre-luminous OSDs (in a single cluster): the
former need MOSDOp v8; the latter will continue to be sent MOSDOp v4.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>