Transaction support with cooperative rebalancing

[svroonland]

In eager rebalancing, we get onRevoked and onAssigned calls in which we can commit in progress messages. But in cooperative rebalancing we may only get onRevoked, at which point we can not perform any commits.

Find out how this relates to the "invalid group generation" errors.

[erikvanoosten]

@svroonland Is this solved by #1434?

[svroonland]

Not that I'm aware or expecting. There's a test where the cooperative strategy is commmented, should not be much work to find out if it works.