This was not fatal but with this change it should recover in a slightly
better way.
Also incidentally fixes the cache refetch potentially being attempted
twice by each of the two background population tasks that use it.
Yesterday after the lazer release there was a bit of a spike in the
number of osu-web requests pointed at `/api/v2/beatmaps/lookup`
specifically. The most likely reason for this is that prior to this
commit, the star rating recalculation was fully performed by the
`BeatmapUpdater.Process()` flow.
This process does full metadata lookups, and while it *will* attempt
to use the local `online.db` metadata cache, it *will* also fall back to
API requests if the local metadata fetch fails. While that means that
the local cache likely saved us from a doomsday scenario here, it *also*
is the case that all of that metadata lookup stuff is *entirely
unnecessary* when wanting to just update star ratings.
Therefore, this splits out only the part relevant to star ratings
as a separate background process, so that it can run completely
locally.
People keep asking why https://github.com/ppy/osu/pull/29553 didn't fix
their databases (as stated in the PR, it didn't intend to), so this
should do it for them.