ActorSystemSpec timeout fixes #3308

So there are two things to this pull request.

* Removal of some unnecessary object creation (causing a bit more GCs)
* Changing the default `ActorRef` collections in `ActorCell` to hash based versions instead of tree based ones since the hash code of ActorRef is fast now.

These two changes makes `ActorSystemSpec` go really fast locally, but it's still fluctuating on the slow jenkins box.