NOTIFPLT-47:  Add NotificationState to NotifictionEntry to track things ...

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https://issues.jasig.org/browse/NOTIFPLT-47

This PR is a re-boot of an earlier one and reflects the discussion there.
Consider specifying the method to return `ImmutableMap` to more strongly communicate to callers that it is unmodifiable (as opposed to their having an unpleasant surprise on attempt to modify the returned `Map` object.)