Currently, usage tracking info is updated when a page is edited. All old usages are discarded, and the new ones placed in the database.
This does not work in a multi-lingual scenario, since:
* upon edit, we only know the usages for one language (the one being rendered immediately upon save, generally the site's nominal content language).
* when re-rendering, we only know the usages for the current language, we do not know which usages can be discarded.
So solve this, we should:
* upon edit, discard all old usage entries. Since it is not always certain whether parsing or saving is done first, we should use the page-touched timestamp to make sure we only purge obsolete usage entries.
* upon parsing, add all new usages from the parser output
NOTE: when re-parsing is triggered by a template edit, we would not discard old usage entries, even if the respective entities are no longer used.