Currently we have many Wikibase client recent changes entries with the very same rc_params values, as these don't depend on the page changed.

In order to allow for de-duplication here, we should move this into a separate table and make rc_params point to it.

This could be done by putting an integer string into the field, pointing to the PK in a recent_change_params table. When purging out RC rows, we would need to check whether the recent_change_params row(s) are still used and if not, they need to be purged along.