Use a cached instance of KafkaProducer for writing to kafka via KafkaSink.

KafkaProducer is thread safe and an instance can be reused for writing every batch out. According to Kafka docs, this sort of usage is encouraged. It has impact on performance too.

On an average an addBatch operation takes 25ms with this patch. It takes 250+ ms without this patch.

Results of benchmark results, posted on github PR.