Bernard's consent review

[martinthomson]

I've made quite few changes based on Bernard's feedback. The following changes are substantive in some sense:

• Consent is maintained now by receiving a valid response. Previously, it was receiving a response to one of the checks that were initiated in the 30s window. This change makes it more robust.
• Requests are abandoned when RTT (+delta) expires or when consent is lost. Requests aren't abandoned by receiving a response to a later request (the unlikely out of order request). This means that there is another timer that runs, in theory (clever implementers will learn that this timer doesn't need to be an actual timer).

Note: Please don't merge this until we conclude the list discussion.