Provide single threaded multiplexing of connections and subscription groups operations

[mensfeld]

Currently we use threads to operate all the subscriptions (listeners). While this is ok, we could take advantage of the event-based API from librdkafka to be able to do almost the same from a single thread. We would have to however ensure that:

1. None of listeners leak into the managing loop
2. They can be managed as they are
3. Their event bus operates independently (as it does now) for events publication
4. They are blocked only in their respective SG when processing

[mensfeld]

Every single waiting flow could yield from a fiber to the managing thread.