cron: specify index for ticket aging query

Somehow on large datasets (like >1M tickets), MySQL can get confused on which
index will provide the best performance. Generally, as systems age, they will
have significantly more closed tickets than open ones. Therefore, it should be
safe to assume that scanning the `status_id` index on the ticket table for
`open` tickets would be the fastest way to arrive at the sort-of short list of
tickets which should need to possibly be aged.