Kafka optimizations

@tylertreat I found your blog post after Sarama's traffic spiked (thanks!) - here are two optimizations which should substantially improve kafka throughput (though I don't have an environment set up to verify that at the moment).

It's not clear to me from the post whether the `Send()` message is expected to be synchronous or not (ie whether it has to wait for a response). If so then the second optimization is invalid, but I would note that asynchronous producing is the "typical" use case of Kafka, so comparing it with other systems based on synchronous performance isn't all that useful.

Feel free to reach out to me with any other questions you might have about Sarama/Kafka.