Discussion #15042

FOR DISCUSSION, see discussion in related topic: https://github.com/akka/akka/issues/15042

In general the question is if we want to make cluster routers take an actor system / cluster or not. There's also a broader question, as this issue is somewhat related to https://github.com/akka/akka/issues/13802 (`nr-of-instances` being confusing), which may point to that cluster routers should become more specialised?

If we don't want to break APIs, this could still be solved, but by setting the actor system "later" from "the inside", but's going to endup pretty ugly I think...

May eventually resolve https://github.com/akka/akka/issues/15042