This addresses the number one performance concern with realm (when
entering song select). Previous logic was causing instantiation and
property reads of every score in the database due to the `AsEnumerable`
specfication.
The most common scenario is switching between schema versions when
testing. This should alleviate the manual overhead of that for the
majority of cases.
For users, this will show a notification on startup if their database
was purged, similar to what we had with EF.
This came up while testing the new realm thread, where `MusicController`
would fall over when `OsuTestScene` changes the global beatmap from an
async load thread (causing a cross-thread realm access).
We don't want to have to schedule every usage of these bindables, so
this seems like a good constraint to put in place.
I can't confirm this works yet, since I'm not sure what the
preconditions are for the `.note` file to be created. What I can confirm
is that a `.note` file hasn't appeared in my game data directory yet.