This is one step closer to sanity in terms of physical files. As per the
comment I have left in place, we really should be checking file sizes or
hashes, but to keep things simple and fast I've opted to just cover the
"missing file" scenario for now.
Ran into this when testing against a foreign `client.realm` by:
- Noticing a beatmap doesn't load
- Deleting said beatmap
- Downloading via beatmap overlay
- Beatmap is restored but still doesn't work
Note that I've kept the logic where this will undelete an existing
import rather than create one from fresh, as I think that is beneficial
to the user (ie. it will still keep any linked scores on restore).
If the operation timed out on..
```csharp
throw new TimeoutException(@"Took too long to acquire lock");
```
..from an update thread, it would not restore the update context.
The next call would then fail on the assert that ensures a non-null
context in such cases.
Can add test coverage if required.
The editor doesn't currently propagate velocity to nested objects. We're
not yet sure whether it should or not. For now, let's just ignore nested
objects' `DifficultyControlPoints` for simplicity.
Note that this only affects osu! ruleset due to the pre-check on
`isOsuRuleset`.