Store actors in ActorSystem

[Bethibande]

Task

• Store all open (non closed) actors created by an ActorSystem in said ActorSystem
• Once an actor is closed it must be removed from the list / map
• When creating an actor and no id is specified, it must be ensured the generated id does not yet exist

Motivation

The ActorSystem needs to store a reference to all actors it created, until they are closed. Currently it is possible to create two actors with the same id, possibly allowing two instances of the same actor to exist at the same time. In order to fix this the ActorSystem will need to have a list of map of all the actors it created and their ids. This will allow the ActorSystem to just return an existing instance if an actor with an existing id is created.

[Bethibande]

Done, if an id is specified when creating an actor and the id is already in use, the LocalActorSystem will now throw a IllegalStateException. In order to prevent random exceptions, there now is a exists(ActorId) function.