Some status notifiers will want to report the status of all builds all at once for a particular buildset, instead of reporting each build individually as it finishes. In order to do this, the status notifier must wait for all builds to finish, collect their results, and then report a kind of summary on all of the collected results. The act of waiting for and collecting the results of all of the builders is implemented via BuildSetSummaryNotifierMixin, to be subclassed by a status notification implementation.