KafkaSinkCluster - rewrite produce responses to not refer to real brokers

[rukai]

rewrite ProduceResponse::node_endpoints and PartitionProduceResponse::current_leader to avoid pointing at the real kafka broker.
The logic can go in KafkaSinkCluster::process_responses.
We should be able to just clear these values from the produce response and then call Message::invalidate_cache()

There are some extra details in: #1665