Send Terminated when trying to watch already terminated actors

Basically what this does, is when an actor terminates, it swaps the mailbox to `DeadLetterMailbox` (new class).

When the terminated actor receives  the `Watch` message `DeadLetterMailbox` will forward it as a `DeadLetter` to `DeadLetterActorRef`. It inspects the message inside the `DeadLetter`, sees it is a `Watch` and sends watcher a `DeathWatchNotification` message.