Improved workers based on gevent

[s4ke]

#149 (comment)

Follow up here

[s4ke]

Since we are introducing celery workers based on gevent, we should also make use of async i/o patterns for event delivery.

see https://chatgpt.com/share/e/673112c4-a0a8-8004-b007-43865a7a63ac for inspiration

Also: Think about maybe making persisting use gevent inside tasks for better i/o capabilities? But this might be harder to get right with database connection pooling.