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.
Bartłomiej Dach
12fa96de25