Migrating from gogo/protobuf to Google's offical protobuf package

Google is releasing [a new Go API for protobuf](https://blog.golang.org/protobuf-apiv2), which breaks gogo/protobuf, but [the project is now looking for new maintainers](https://github.com/gogo/protobuf/issues/691).

We may need to switch from gogo/protobuf to Google's official protobuf package to keep up with our dependencies and/or the protobuf ecosystem around us.