Allow multiple actors to share an inbox

[Thomasdezeeuw]

It's a rough idea right now but it's fairly simple. In a scenario where the sending side produces a lot of work it might be desirable to scale the consuming actors. For this it would be easy to use an ActorGroup, but that has some limitations (at the moment) where sending of message it's supported that well. So, this issue suggest a new type or possible integration with ActorGroup that a single inbox is shared with a group of actor that all read from the same inbox. This ensures that if an actor is stopped (and not restarted) the other actors can pick up the messages from its inbox.

For now this can also be implemented by using a relay actor that simply send the message on to a group of actors, making that actor responsible for managing the group as well.