Increase or decrease the number of routees in a Pool.
It may be handled after other messages.
Positive change will add that number of routees to the Pool.
Negative change will remove that number of routees from the Pool.
Routees are stopped by sending a akka.actor.PoisonPill to the routee.
Precautions are taken reduce the risk of dropping messages that are concurrently
being routed to the removed routee, but it is not guaranteed that messages are not