Put Kafka 0.8 behind a profile

Kafka does not support 0.8.x for Scala 2.12. This code will have to, at least, be optionally enabled by a profile, which could be enabled by default for 2.11. Or outright removed.

Update: it'll also require removing 0.8.x examples, because otherwise the example module has to be split.

While not necessarily connected, it's probably a decent point to declare 0.8 deprecated. And that means declaring 0.10 (the other API left) as stable.