fix: do not create many redis subscriptions per token - fast yielding background

[benedikt-bartscher]

@masenf any idea why fast yielding background tasks seem to lock up after some state updates?

[benedikt-bartscher]

@masenf i accidentally pushed unfinished changes and forgot to mention that the test needs to be run with redis

[benedikt-bartscher]

I pushed 389f4c7 which fixes the issue. Another idea to improve this even further: one shared subscription per token and worker (if I am not mistaken, we basically need to lock a token to a worker on redis, further locking can be done with asyncio). Reuse the subscription and release/garbage collect it after a delay.

However, I would merge this as-is, because it works and does not change the behavior much. I would then try implementing the other idea in a followup.