Events stream queries should filter on sample_id

[fbertsch]

We did some investigation on events_stream, and filtering on both sample_id and event_category significantly improves performance: https://gist.github.com/fbertsch/f560bb0c182b3544c640fcf7a2425a7f

We should add those two filters into the query. The sample_id filter is straightforward, but the event_category will have to come from the event itself.