Support for multiple servers

[woile]

Kafka accept n + 1 servers to bootstrap initial cluster metadata.

https://aiokafka.readthedocs.io/en/stable/api.html#aiokafkaconsumer-class

[tomchristie]

First question would be what we want the Broadcaster.__init__ interface to look like to support this.

Do we want to change it to Broadcast(url: Union[str, List[str]]), or something else?

We'd need to raise errors if the urls included mixed backend strings, or if multiple urls were used with a backend that only supports a single hostname for connections.