Fetch tag definitions, mostly meaning the facets based on the hierarchy in UMTHES

This turns the stored tag definitions into a unified cache for the primary information (title and parents) as well as the derived facets (meaning the expanded path to the top-level tags) using the same age-based expiration as in !107 (merged).

This is a bit complicated insofar updating a single tag might mean that its ancestor tags need to be fetched and hence updated as well while making sure that the secondary updates always use information cached during the primary update.

Closes #114 (closed)