Refactor rend_cache_store_v2_desc_as_dir and rend_cache_store_v2_desc_as_client to avoid duplicate code

There's a lot of overlapping code here in terms of checking the timestamp on the parsed descriptor, checking if we have a newer one already, creating and linking the new entry, etc. It's not exactly the same but a lot of it sure is similar. This is the sort of situation where a subfunction to do the shared behavior might be handy.