Process finish due proxy garbage collection should trigger shutdown

For memory-safety reasons, every actor is automatically finished when its owner manager gets garbage-collected (from multiprocessing.managers.BaseManager implementation).

We are exposing the Actor.shutdown hook to be called because of main proxy ActorProxy.__exit__ or any ActorProxy.shutdown calls, but it is not when process is finalized via garbage collection.

Really not an issue, but it's counter-intuitive. Calling Actor.shutdown during garbage-collected finish should be considered.

Considerations

• Changing current behavior will render __del__ method nearly useless (it became safe in py34), so maybe we should update the documentation and take no action.
• Calling shutdown twice because of ActorProxy.__exit__ and ActorProxy.shutdown is currently ok-ish, since ActorProxy.shutdown calls are explicit and the implementer would be the one to blame here, but any extra Actor.shutdown call (after either ActorProxy.__exit__ or ActorProxy.shutdown) caused by garbage-collection should be avoided by extra logic. Since both methods should already finish the process itself, this may not require any specific code (on the actor process side).