Daily update of POFile statistics should process smaller set

A full run of POFile statistics updates takes over 10h on our production system. It's even slower on staging, so we are unable to properly test it there.

We should run frequent statistics regeneration and verification on a smaller subset of all the PO files, say, those touched in the last 7 days. Care should be taken to update all the POFiles from the same sharing equivalence class, since even if they were not directly touched, statistics in them might have been affected.

This is an improvement of the approach in bug 409330 which does a cut-off by the smallest POFile DB ID, which is pretty ugly, and doesn't take sharing into account.