Kafka Source should split very large partitions

If a single Kafka partition has a very large number of messages, the map task for that partition can take a long time to run leading to potential timeout problems. We should limit the number of messages assigned to each split so that the workload is more evenly balanced.

Based on our testing we have determined that 5 million messages should be a generally reasonable default for the maximum split size, with a configuration property (org.apache.crunch.kafka.split.max) provided to optionally override that value.