[k2] top-level wire protocol spec and encoding implementation

[neonphog]

This spec should include:

• the encoding format (messagepack)
• the exact encoding parameters (structs are maps, names are camelCase or whatever we agree on)
• a method for rejecting connections on implementer defined whim (i.e. holochain switch to dht v2, so it's no longer compatible)
• preflight validation (prove we have at least one space in common that we are both allowed to speak on)
• send/recv arbitrary opaque specific module implementation data. I.e. not just "the dht module" we need to know to send it to the actual "dht_v1" module.