Remove "actor are stopping when they detect there are no more messages" functionality

Right now Actor stop as soon as they detect no one will ever send them a message.

This is done by checking the refcount over their mailbox. Once it reaches 1, it means only their copy (in the actor context) exists and no new message will come.

While this sounds neat, it implies a lot of papercuts to enforce.

For instance,

• The actor handle does not expose the actor mailbox directly. (That would break our refcount mechanism)
• Therefore spawning returns (Mailbox, ActorHandle)
• Self scheduling breaks the logic, therefore we still need to send a termination message.

The suggestion is to remove this functionality and force the chain of actor to send a quit command downstream.