Consider exposing step information/counter in exchange protocol messages

[dlongley]

Determine if it is required/of use to include some kind of step counter in exchange protocol messages -- for more complicated, multi-part exchanges. It might also be of use for clients to be able to indicate the step number they are expecting when making a request, as it's conceivable that the same VPR might be sent for consecutive steps in a complex exchange. The alternative is to require different VPRs for consecutive steps to avoid this (this might result in a simpler protocol and cover all the use cases, with the trade off of being less explicit).