Stale member should trigger onPartitionsLost when leaving group

When the poll timer expires, the new consumer proactively leaves the group and clears its assignments, but it should also invoke the onPartitionsLost callback. The legacy coordinator does the following sequence on poll timer expiration: send leave group request (here), invoke onPartitionsLost, and when it completes it clears the assignment (onJoinPrepare here).

This issue is most probably what is causing the failures in the integration tests that fail expecting callbacks when the poll interval expires (like https://issues.apache.org/jira/browse/KAFKA-16008)