sub-protocols

[ghost]

There is already some interest in using cable for non-cabal chat purposes as well as questions about how to layer other features such as sharing files on top of cabal with cable. This could be done by adding more message types, but that would require more coordination (each protocol needs to reserve message type varints for each of its message types) and integration than allowing for sub-protocols where each sub-protocol only needs to reserve a single sub-protocol varint. Using sub-protocols would also let cable focus more on low-level concerns such as routing and cryptography while more application-specific tasks such as text chat can be versioned separately. This is already somewhat the case with the post vs message parts of the cable spec, but using sub-protocols would also move the query logic into a separate realm from the networking and hash/data delivery parts of the protocol.

work-in-progress branch for sub-protocol refactoring: https://github.com/cabal-club/cable/tree/protocol