fix kafka benchmarks

[rukai]

kafka benchmarks are currently quite wonky.
We currently use the cpp rdkafka driver which does not map well to rust async.
Things to do from here:

• add benches using https://github.com/influxdata/rskafka this will give us a point of comparison with a driver that properly uses async.
• Need to use unique keys to ensure spread across topology3 cluster, otherwise it all gets routed to a single node 🙃 - Windsock kafka fix routing #1417
• rdkafka maintains a queue of outgoing produce requests which maps poorly to rust async. In order to alleviate this we can:
  • use multiple producer objects to get multiple polling threads? not sure if this works like that.
  • do not start the request timer on produce requests that fail to queue
  • Maybe dropping down to BaseProducer and implementing our own threaded logic would be better?