Control over payload decode

[nick-zh]

For the next major we should change the current decoding logic.
Right now the message is immediately decoded, this should be an additional seperate step, triggered by the application.
The reason of Kafka headers is, that you can make quick decisions without decoding the payload.
With the current version of this lib, we always decode the payload and don't leverage that advantage.
Although the price for decode is small, this is of significance for topics with millions of messages.