Whether the aggregator has reached a quiescent state. This will happen at some point after 
            IndividualAggregator.prepare has successfully completed for the 
            aggregator. An aggregator is in a quiescent state when all the PersonaStores 
            listed by its backends have reached a quiescent state. Once it's reached a quiescent state, this property will never change again (from 
            true to false).
It's guaranteed that this property's value will only ever change after 
            IndividualAggregator.is_prepared has changed to true
            .
0.6.2