Enable configuration of Kafka idempotent producer in output

[nicktelford]

Kafka's idempotent producer ensures that only one copy of a record is written to the Kafka broker, even in the face of network failures. Sarama supports this feature.

I'm not sure how it's done in Sarama, but usually you just need to configure a transactional.id, which uniquely identifies distinct client instances across restarts.

Relates to #1204.