Overhaul deliveries, simplifying implementation and adding test

[longshorej]

• Mailbox no longer needs to be concerned with cancellable messages
• A delivery is stored locally in the actor's context
• A system message is scheduled indicating a delivery (with a name and id) should be delivered.
• When received, if the local id for that given name matches the id, i.e. it hasn't been superceded, the local msg is delivered