This will allow us to move some stuff to be more async; this commit
however is entirely mechanical. We have to refactor a bit because
the Monitor locks did allow for us to re-enter, but we won't be able
to do that with SemaphoreSlim.

When the project system is loading a lot of projects at once, we can
end up with a bunch of threads all contending for the main
VisualStudioWorkspace lock, since we ultimately acquire the singleton
lock once we're updating CurrentSolution. This allows us to be async
so we aren't blocking up a bunch of thread pool threads.