I started experimenting with it on https://github.com/ktoso/akka/commits/wip-3623-plus-eventstream-actorclassification-unsubscribe-ktoso very WIPy still, will ping once it's worth looking deeper.

One question I have though, is if we're OK with `ActorClassification` spinning up the unsubscriber during creation? Or should we expose the init work to end users? Thanks in advance for hints :)