Design negotiation mechanism for prop324

Based on discussion with @mikeperry, I'm going to try to get the negotiation mechanisms for prop324 (rtt congestion) figured out this week.

AFAIK what needs to get agreed upon between the client and the relay are:

• Will be we using xon/xoff or stream sendmes?
• What is the circuit SENDME window?

Also we may need to figure out how an onion service advertises support for this protocol.

(anything else?)