Query all PersistentActor identifiers, i.e. as defined by the `persistenceId` of the PersistentActor. The stream is not completed when it reaches the end of the currently used `persistenceIds`, but it continues to push new `persistenceIds` when new persistent actors are created. Corresponding query that is completed when it reaches the end of the currently currently used `persistenceIds` is provided by CurrentPersistenceIds.