Rewrite AtLeastOnceDeliverySemantic

[Aaronontheweb]

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Major changes needed:

• Remove ImmutableDictionary<T> in favor of Dictionary<T> - if the semantic isn't accessed concurrently, then it's not needed.
• Add ability to add single UnconfirmedDelivery messages to the AtLeastOnceDelivery state incrementally, so these items can be retrieved via Recover<UnconfirmedDelivery>.

[Aaronontheweb]

Looks like the dictionary doesn't need to be sorted either - doesn't offer much value in this instance since we need to scan all of the entries in the _unconfirmed whenever we are computing redelivery.

https://stackoverflow.com/questions/2376459/unexpected-poor-performance-of-sorteddictionary-compared-with-dictionary