In preparation for msgr2, define internal messenger <-> protocol
interface (as opposed to external messenger <-> client interface, which
is struct ceph_connection_operations) consisting of try_read(),
try_write(), revoke(), revoke_incoming(), opened(), reset_session() and
reset_protocol() ops.  The semantics are exactly the same as they are
now.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>