[WIP] Unite MessageToSend and ConsumerEvent

[wvanbergen]

Start of #269.

1. New names: ConsumerEvent becomes ConsumerMessage and in the producer, MessageToSend becomes ProducerMessage. Once we're ready to unite it I'd like to simply call it Message, but right now that is in used internally for messages between client and broker. Can we find a new name for this, e.g. BrokerMessage or ProtocolMessage?
2. Remove encoder interface, and simply use []byte for keys and values.

Still to resolve: Metadata, and Err.

[wvanbergen]

@mkobetic @Shopify/kafka

As discussed in out walkthrough of the new consumer.

[wvanbergen]

Going to close this as this is outdated and I am not so sure about the approach anymore, I'll open an issue instead;