ceph: include transaction id in ceph_msg_header (protocol change)

Many (most?) message types include a transaction id.  By including it in
the fixed size header, we always have it available even when we are unable
to allocate memory for the (larger, variable sized) message body.  This
will allow us to error out the appropriate request instead of (silently)
dropping the reply.

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