Track remote client version info to prevent communication incompatibilities

In the discovery mechanism we should transfer information about possible per-client differences.

I believe the most simple solution would be to delegate this to users. They could define a `net-version` value that increments per each breaking change. Any Nimbus client receiving messages that sees the `net-version` above their own will reject the message and log a warning.

If users do not wish to bother we can exclude this header information all together. JChannels allows easy modular header management, so this should be pretty easy.