In bug 1346567, we managed to re enable the expiring algorithm for perf data.
It's main mechanics are working up to this day. We're now keeping the
performance_datum table at ~300 million rows (while previously it peaked at >750 million).
However, there are other tweaks we could employ to further expire useless perf data. So we have more decent sized tables (performance_datum, performance_signature, performance_alert_summary).