Expose Kafka socket parameters, max message size.

@pgarbacki

One more PR for you.  It's important to be able to set these values for the client, especially the max message size (because if the server uses a size larger than the default, the consumer will fail).
Would you mind setting those either to the kafka default values or leaving them empty in the repository?

I should have done the same for kafka.rebalance.max.retries.