avoid "no results" alert when there are results

Created by: camdencheek

When streaming results, the aggregator does not hold the list of results in memory, so the alertObserver was being created with hasResults == false even though we've streamed results. That caused us to show a "no results" alert in some cases even though we've sent results.

This commit adds a resultCount field to the aggregator so it can track how many results it has streamed, and use that for creating the alertObserver and for generating alerts.

An alternative here would be to add a result count to the Stats struct, but it felt strange to be updating that when no stats event was sent down the stream.