This PR is intended to have no impact on existing sync servers. The fields and enum values in the protobuf have been named such that their JSON equivalents match the existing constants we have in the codebase.
Adding this provides a few benefits:
1. The protobuf serves as canonical documentation of the protocol in a form that's much easier to read than the existing code.
2. Protobuf parsing of JSON is likely to be better than our hand-written version.
3. We can (in a later PR) add a configuration option to use binary encoding instead of JSON, saving network during syncs.
4. Servers written in other languages are easier to write and update as time goes on, especially as we extend the protocol.
