Put network protocol version in the demo header

[sprunk]

See #2140.

Something to figure out would be how to tell autohosts this, since either modifying SERVER_STARTED or sending a different packet would be a breaking change. Perhaps just send two packets (SERVER_STARTED as-is, plus a new future-compatible one with metadata)?

[p2004a]

This change looks good. For autohost I would change the SERVER_STARTED adding a field to it. It is one breaking change and IMHO that's the point: we switch from "magic frozen autohost interface" to "able to make changes and communicate".

[sprunk]

Sounds good, will do.

One more thing, protocol versioning was mentioned in the context of #2712 which doesn't introduce any breaking compatibility. If we increment the number on non-breaking changes then it becomes hard to know when you should stop trusting existing packets. Should this be two numbers, major (incremented on breaking changes to existing packets' meaning) and minor (incremented when it's just adding something new but all existing packets still mean the same thing)?

[p2004a]

I have a hard time at the moment determining where would be the breaking/non-breaking line in context of protocol. It's unlike breaking/non-breaking in context of libraries where you can just not use new feature. Any change, any addition could be considered breaking from perspective of application interpreting protocol, e.g. addition of new chat message type can be considered breaking from perspective of application that extracts all messages for moderation purposes.