NullCallbackStore in action

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An example case when using the NullCallbackStore might be when callback objects A, B, and C are stored in memory because there is enough free memory. Then when callback D comes, the persistence policy is triggered and the NullCallbackStore is asked to persist callback D. The NullCallbackStore will throw away callback D and create a FailedCallback object to take its place. Then callback E comes, and there is still too little free memory, so that is thrown away by the NullCallbackStore. Then the client calls to get its callbacks. It will receive a List containing callbacks A, B, C and the FailedCallback. When the client asks the FailedCallback for its callback payload, it will throw the aforementioned exception.