Set Kafka isolation level

[zentol]

Converted https://github.com/immerok/recipes/pull/2 to an issue.

I faced an interesting challenge not a while ago where duplicate records arrived at non-idempotent sinks while using a similar code structure to the one provided in the kafka-exactly-once recipe. At the end of the day, one realized that this was due to using the default isolation level for the Flink KafkaSource, which is read_uncommitted.