Reduce object creations in in the poll method on the Kafka consumer

This is what the Kafka consumer does https://github.com/getsentry/rust_arroyo/blob/main/src/backends/kafka/mod.rs#L136-L154

It creates a topic and a partition object for every message. We should not need all this overhead. Find a way to create only the message (even that could be questionable, but that is a bigger change) and use references to topic and partitions owned by something else.