Only the file related ones are really required outside of tests, but
seems like as good an opportunity as ever to get the rest of the models
into the game project.
Due to the lack of locking, there was a chance the the update thread
`context` was retrieved just before the `flushContexts` call, followed
by `.Refresh()` being run while the blocking behaviour was invoked.
This can be seen in test failures such as
https://ci.appveyor.com/project/peppy/osu/builds/39859786/tests.
As an aside, I tried multiple different methods to avoid `lock()` on the
update thread but they felt flaky. The overhead of lock when there's no
contention is reportedly around 30-50ns, so likely not of concern. We
can address it at a later point if it becomes one.