Fix kafka consumer not destroyed in close

[ikeberlein]

Not destroying underlying kafka consumer in close() method leads to unclean librdkafka shutdown and very often to segfaults.

[arnaud-lb]

Normally the rk handle is destroyed when the KafkaConsumer object itself is destroyed, so I don't think we need to close it explicitly here. However maybe we don't destroy various resources in the right order when the rk is closed later?

Do you have a reproducer?