Code generation for messages

Nobody has claimed this yet.

Currently we use a mix of encodings: JSON to wrap Golang gob-encoded messages.

* The gob encoding is not portable. While theoretically documented in https://golang.org/pkg/encoding/gob/ , in practice gob encoding restricts usage to Go-based clients only. We should use a more portable serialization format so nodes can be developed in other languages.
* Consider whether we should keep the current JSON wrapper: with a typed, schema-based serialization (capnp, protobufs), we can rely more on code generation, and do less manual handling for each message, reducing the amount of boilerplate (eg, `data.go`).
* Fine grained versioning built into the networking protocol, assisted by the protocol compiler. https://capnproto.org/language.html#evolving-your-protocol

Start by reading data.go and tracing how the networking protocol currently wraps and decodes gob messages. Compare the issue's proposed capnp or protobuf approaches, then define a selected portable schema, generated message handling, and protocol versioning that supports non-Go clients.

Written by the indexing model from the issue text.