Race condition between subscribe and stream leading to duplicated records

[backuitist]

When subscribing to a topic the KafkaConsumerActor adds a rebalancing hook that registers FetchRequests.
Then when calling stream the KafkaConsumer will requests partition assignments and - that's the race condition - depending on the timing might get some partition ids for which it will also register FetchRequests.
This is aggravated by the fact that the KafkaConsumerActor.State does not restrict the amount of FetchRequests per (partition, consumer), leading to duplicated records sent downstream.

[vlovgr]

Thanks for finding and reporting this @backuitist! I found another related issue, and opened #106 to address this. Would you mind having a look?