Pubsub: Use new broadcaster disconnection bugfix (conditioned with a feature flag)

[roekatz]

Make pubsub server compatible with the new fastapi_websocket_pubsub api. This is important in order for fastapi_websocket_pubsub to be able to wait on the broadcaster's reading task, and bailing out on connection loss.

[orishavit]

Waiting for release of fastapi-websocket-pubsub