DB analyzer: sort slow queries with better heuristics

[ankush]

I think it's sorted by rows examined, which isn't necessarily a good choice here. Slow query log should be sorted with duration * calls = total_duration IMO.

E.g. First query is rename, that happens rarely and not worth optimizing VS the session query happens frequently and 192 seconds is insane amount of time to spend on that simple query.

[ankush]

Also, we can let users pick a different sort order.