Integrating Kafka partitioning scheme with Voldemort cluster.xml

- Rename KafkaConsumerDefinition to KafkaTopicDefinition as it will be used by both client and server
- Read from cluster.xml to initialize the VeniceStore
- Add consistent partition, serializing classes for Kafka to use
- Adding KafkaRoutedStore object for producing objects into Kafka
And perhaps leave some comment about our assumptions of what type of key will be passed in by the Kafka producer.