Allow user to commit offsets without handling response when using committable sources

[seglo]

Short description

Allow the user to commit offsets back to Kafka asynchronously without waiting for a response.

Details

The Committer.batchFlow will use mayAsync to backpressure offset commit callbacks. When we call commitScalaDsl we will use the Ask pattern to ask the KafkaConsumerActor to commit the offset map immediately (using KafkaConsumer.commitAsync) and reply with a response when the commit response is handled during a subsequent poll. This implementation has the benefit of making sure we lose as few messages as possible in the event of a failure while using at least once message delivery, but it can also considerably slow down the whole consuming stream. A compromise would be to commit and not wait for a reply which would allow the stream to process records faster, with the tradeoff of potentially reprocessing more records after a failure or rebalance.

It would be nice if there was a way to "fire and forget" the commit so we don't back-pressure in the Committer.batchFlow waiting for replies. We also wouldn't create the overhead of additional callback threads.

[seglo]

FYI I'm going to pick this up!

[ennru]

Implemented with #883