Add support for message cancelation

Requester is allowed to cancel an outstanding messages via xio_cancel_request
the result of the cancelation shall be trigered by on_cancel callback.
Responder is triggered by on_cancel_request callback to cancel an outstanding request.
it should respond the request with xio_cancel callback.

Signed-off-by: Eyal Salomon <esalomon@mellanox.com>
changing variable name
Signed-off-by: Eyal Salomon <esalomon@mellanox.com>