Do not commit offsets for past generations if partition not owned

[nachogiljaldo]

This is a skeleton PR aiming for discussion related to #1308

In that issue, I describe how rebalances can lead to a situation where the consumer receiving the partition does not read messages in spite of lag existing (because conn's offset is > than the broker's current offset and there's no new messages coming).

I was aiming to fix it by avoiding commits for past generations, to do so, I add the generation id to the message and when committing, I log an error instead of adding it to the stash if it belongs to a generation < than current.

This has the risk of losing valid commits when the new generation comes (as its associated generation < latest generation.id), however it works because a new generation ends up creating new connections which means uncommitted events are duplicated. It is not that I love it because it works incidentally but it does work as my test shows.

If you have a different / better approach I am happy to explore it if you do explain it a bit.