MemoryStore: Provide ChangeFeed

[bartelink]

In order to be able to integration scenarios where one is running a Reactor or Projector driven by events being added to a store, the MemoryStore should provide an interface that emits events as soon as they as committed.

• Presumably as an IObservable?
• Needs unit and integration tests in this repo
• Probably should be integrated into Equinox shipping : container finalization process manager dotnet-templates#56

See a diagram that pretends this already exists
related, packaging up Consumers for it in Propulsion: jet/propulsion#64