Do a final commit on end consumer group generation for immediate commits

[stevevls]

This is a WIP and isn't ready to commit until I put a unit test together. 😄

This PR adds a final commit upon exiting the consumer group when using the synchronous commit mode. Otherwise, it's possible that we lose commits on generation end (or other events that initiate the end of a generation such as closing the reader).

Fixes #711

[rhansen2]

Possible related to #772

[stevevls]

Good call. This definitely could be related since they are not setting the CommitInterval. I think most users (myself included) don't realize that the commit loop has to be explicitly enabled.

[rhansen2]

From https://docs.confluent.io/platform/current/clients/consumer.html

Offset commit failures are merely annoying if the following commits succeed since they won’t actually result in duplicate reads. However, if the last commit fails before a rebalance occurs or before the consumer is shut down, then offsets will be reset to the last commit and you will likely see duplicates. A common pattern is therefore to combine async commits in the poll loop with sync commits on rebalances or shut down. Committing on close is straightforward, but you need a way to hook into rebalances.

Sounds like this would be a nice addition to the Reader.

@stevevls Are you interested in continuing this PR and adding test? If not I could give it a crack.

[derekbassett]

If this PR is not ready for review might make sense to make it a Draft PR. @rhansen2 I think this makes sense for you to take over since it's been several days.

[achille-roussel]

🚢 it!

[stevevls]

Thanks for landing this team! 🙌 I'm sorry I didn't see the mention for whether I wanted to keep working on it.