Add Persist and Recover messages for unconfirmed deliveries

[Aaronontheweb]

Need to add an Akka.Persistence.IMessage type for a confirmed delivery which can be recovered during replay.

[Aaronontheweb]

This is our proposed state persistence strategy

[Aaronontheweb]

And subsequent automatic recovery strategy

Currently, the design of the AtLeastOnceDeliverySemantic doesn't allow for the possible recovery of individual UnconfirmedDelivery messages. Its interface is designed only to allow "all or nothing" recoveries - you either get the entire state or none. This is why #4 is necessary.