State container

[rogeralsing]

This PR is much like @HCanber's #766

It's for review and memory footprint optimization.

In this PR, the state found in ActorCell.DeathWatch.cs + the _behaviorStack has been abstracted out to a state container that by default only holds a WatchedBy, that is, it's parent.
All other state is lazily allocated if the actor needs more than that initial state.

The actor size have now gone from 1334 bytes to 1144, which is about 16% less memory per actor.

Thoughts on this?

[stefansedich]

@rogeralsing I like this, nice and simple and easy to follow.

[rogeralsing]

So, anyone else had a look at this?

[Aaronontheweb]

I have not - will take a look after .NET Fringe craziness settles down on Tuesday

[Aaronontheweb]

So if I understand this correctly... IActorState encapsulates the BehaviorStack, deathwatch, and child containers and lazily builds it out. One thing I don't quite understand yet (I had some trouble reading the diffs for some reason - Github issue) though is does each piece of the actor state build lazily? I.E. if I subscribe to deathwatch do I automatically get a ChildContainer too?

[rogeralsing]

The ChildContainer stuff is separate from this PR.
But child container starts out in a "empty container" state, very similar to what I do with the other state in this PR.

Anyway, the DefaultActorState here is the minimal startup state.
It can handle one WatchedBy and one Receive behavior.
That is, what all actors have at start up, they have a default behavior and they are watched by their parent.

Many actors never leave this state, they might be workers that never use BecomeStacked and are never watched by anyone else than the direct parent.

But if they are, they will transition to use the FullActorState transparently, which enables having multiple watchedby, watching and a full behavior stack.
Once in this state, the actor will never revert to the DefaultActor state.
We are just trying to optimize memory footprint for the default case.

Does that make sense?

[Aaronontheweb]

Ok, in that case then I think we should merge it in and get the memory footprint down.

[Aaronontheweb]

@rogeralsing merge when you're set