Put multiple messages for the same (topic,partition) into the same message.
Put multiple messages for the same (topic,partition) into the same message.

This fixes KafkaClient.send_messages() with multiple messages for the same
topic and the same partition.

jedisct1/kafka-rust,spicavigo/kafka-rust,frew/kafka-rust,Lingling7/kafka-rust,spicavigo/kafka-rust