Fix rdkafka consumer hanging when disconnecting (#13144)

Calling `consumer.close()` was hanging indefinitely when the
`optimizedRebalance` config was enabled.

The issue is the same as
confluentinc/confluent-kafka-go#767

The `rebalance` method needs to call `consumer.unassign()` when closing.
It was not doing this because `this.consumer` was being set to
`undefined` before disconnecting.