Extend protocol to permit request cancellation

[daviddawson]

It should be possible to request the cancellation of an ongoing request.

This is not an error state, it is the client deciding that it has waited long enough, and so now wants to terminate.

This should be in the form of a new client step that closes the channel. The server side should interpret this to cease work on what it is doing. It may be that this is treated as a courtesy by the server and so is ignorable.