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.
This reverts commit f3faad74d5, reversing
changes made to 712e7bc7bf.
Several issues arose after migrating to 5.0, including, but possibly not
limited to, performance regressions in song select, as well as failures
when attempting to save beatmaps after metadata changes in the editor.