code: function(params, messages, cb) {
      var correlationId = _cbt.put(prodResponse(cb));
      var msg = envelope(prod.encode()
                             .correlation(correlationId)
                             .client(_options.clientId)
                             .timeout()
                             .topic(params.topic)
                             .partition(params.partition)
                             .messages(messages)
                             .end());
      sendMsg(msg, correlationId, cb);
    }
docs: write a produce request to kafka
  
  params:
  topic: the topic to write to
  partition: the partition to write to
  
  messages: an array of messages to write to kafkia, messages may be
  - a string
  - an array of string
  - an array of objects of the form {key: ..., value: ...}
  if key value pairs exist the key will be written to kafka for reference purposes
  otherwise a null key will be used