Introduce versioning to Kafka messages

[simonredfern]

Suggestion for versioning of Kafka messages:

After input from Ulrich, send version with the message - so we don't rely on the transport (Kafka)

{"version" : "k0.1.0"}

Use "k" to denote that this is separate to the api version (v)

if k0.1.0 is requested, then we expect the answer to be in k0.1.0