fix scheduling logic for past data aggregation tasks

With the addition of the insert_time_slice column, we can now have multiple
index entries for a {column_time_slice, start_schedule_id} pair. We can reduce
those into a single index entry and submit an aggregation task for that
combined entry.

PastDataAggregator also now has a deleteCacheIndexEntries function to take care
of deleting all index entries for a {column_time_slice, start_schedule_id} pair.