Add logical connections to UDP RFC

[gdamore]

The UDP RFC lacks support for logical connections, which means that there are SP protocols that cannot be used with it (any that require a backtrace, like REQ/REP), and that it cannot be used in device constructs.

We need the ability to create and manage logical connections. Furthermore, we need protocol support for keeping these active, to ensure that we tickle firewalls etc. that may use UDP state tracking to keep ports open or NAT mappings active. And like those, we need a way to detect an idle session and clean it up.

The answer is, unfortunately, that we have to build a stateful connection layer on top of UDP. Fortunately it doesn't need to provide most of the overhead of TCP -- we don't need reliable delivery, we don't need guarantees against reordering or message alteration, or duplication.

Much of the protocol can be done in a stateless/idempotent manner, using just four lower level message types:

• DATA
• REQUEST_CONNECTION
• ACKNOWLEDGE_CONNECTION
• ERROR

I've updated the RFC to deal with the particulars.