Peformance optimization: only clear parser cache when page contains parser function

[JeroenDeDauw]

We clear the parser cache upon page creation, so our parser function can immediately access the newly generated and saved persistent ID, since that ID is not yet saved on the first parse. Ideally we only do this on pages where the parser function is present.

This is a follow up to

• https://github.com/ProfessionalWiki/PersistentPageIdentifiers/pull/46