libceph: abstract message data

Group the types of message data into an abstract structure with a
type indicator and a union containing fields appropriate to the
type of data it represents.  Use this to represent the pages,
pagelist, bio, and trail in a ceph message.

Verify message data is of type NONE in ceph_msg_data_set_*()
routines.  Since information about message data of type NONE really
should not be interpreted, get rid of the other assertions in those
functions.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>