Improve Kafka Reply Topic Logic in the KafkaProducerMessageHandler

[garyrussell]

Currently:

		if (replyTopic == null) {
			if (this.replyTopicsAndPartitions.size() == 1) {
				replyTopic = getSingleReplyTopic();
			}
			else {
				throw new IllegalStateException("No reply topic header and no default reply topic can be determined");
			}
		}

However, the ReplyingKafkaTemplate could have already determined a default reply topic - this will need a change to the template to expose its default (if present).

The current mechanism (determineValidReplyTopicsAndPartitions()) does not work because the partition might not have yet been assigned.