KafkaProducer011 seems to leak threads when not in exactly-once mode

KafkaProducer011 appears to be leaking kafka-producer-network-thread threads.  As far as I can tell it happens when it is not in EXACTLY_ONCE mode, it seems that it creates a FlinkKafkaProducer but never closes it, even when the FlinkKafkaProducer011 itself is closed.

I observed this when running a local cluster and submitting and then cancelling a job, a lot of kafka threads were left alive afterwards.