kafka alerts sent sequentially

Currently the kafka alerts work that is going to be merged runs one kafka worker with the kafka bootstep, but this worker has a concurrency pool of 1. This means that even though the kafka alert send tasks are written to be parallel, they will be completed sequentially. Under ideal operating circumstances the latency difference between alerts is probably in the noise, but if a broker goes down and we don't catch it before trying to send an alert then that broker may block the send tasks indefinitely. We can fix this either by adding one kafka worker per broker that we want to send alerts to (currently Deep and Leo's preference), or by increasing the kafka worker's concurrency (my slight preference).