Proposal: Add additional reliability guarantees

[luna-duclos]

I'd like to propose adding two additional types of reliability guarantees to CurveCPR, which could be negotiated using two extensions.

The first type would be a fully unreliable connection, where no attempt at reliability, acknowledgement or congestion control is done.

The second is a sequenced stream, where in the face of packet reordering, only the latest packet is kept, and older ones are discarded, again, with this, no attempt at extra reliability, acknowledgement or congestion control is done. The idea is that the data is so time critical that any resending would yield outdated data anyway as fresher data would've been sent.

I was hoping for some guidance on where the code changes for this would be, as well as feedback on whether or not this belongs in curveCPR itself.
I've investigating implementing it purely in the callbacks, but I found no way to implement it cleanly without screwing up chicago or the reliability code.

[luna-duclos]

As a clarification: I'll very much be implementing these myself, but I'd like some feedback first :)