A Go module's .mod and .info for a canonical version, and RPM repodata
files named after their checksum, never change once published. They
went through the metadata TTL, so each cost a revalidation every TTL with
cache_metadata on and a full fetch on every request with it off.
They are now kept in the metadata cache with or without cache_metadata
and served whatever their age. A stored copy is checked against its
digest and fetched again if it doesn't match. Go queries, @v/list,
@latest and repomd.xml stay on the TTL.
