Wrap ConsumerGroup.onRebalance to avoid waterfall bug (#922)
Wrap ConsumerGroup.onRebalance to avoid waterfall bug (#922)

If the onRebalance function yields a value, it would be reflected
into the waterfall and move the callback to the 2nd parameter.

see #918
SOHU-Co/kafka-node,SOHU-Co/kafka-node,SOHU-Co/kafka-node