Self-referential actors will self-terminate

[chasminimus]

If an actor only knows itself (i.e. its owners set only contains itself), it will automatically terminate. We don't currently check if that actor is still actively sending messages to itself/doing work. We need to add some kind of message counting to make sure that there aren't any in-flight messages to itself. Look into inspecting the mailbox and comparing it to the messages sent.

[dplyukhin]

Deliverables

• [ ] gc.ActorContext should keep track of how many messages they have sent using each reference. (This can be implemented as a map from Tokens to integers.)
• [ ] gc.ActorContext should keep track of how many messages have been received using each reference. (This means that each AppMsg should include the token of the reference used to send that message.)
• [ ] Actors should terminate if self is their only owner and there are no pending messages from self.