Protocol versioning

[pvlugter]

Add cloudstate protocol versioning, so that we can align compatibility between the proxy and the language support modules. Ideally both sides can support multiple versions and negotiate on the latest supported, during discovery.

[viktorklang]

There should already be versioning in the protocol: https://github.com/cloudstateio/cloudstate/blob/master/protocols/protocol/cloudstate/entity.proto#L172

[sleipnir]

yes it exists but is totally ignored by all supported languages, including java and node

[marcellanz]

No semantics are defined yet, we have to spec it! Lets work on #119 :)
I'd like to have a spec as promised by the project, a "A standards effort" (spec) and a "A reference implementation" \o/