VisualStudioRuleSetManager acts a single global cache for the contents
of rule set files, and also offers eventing if the effective contents
change. This was previously only being called on the UI thread
everywhere, but it won't be much sooner.

Now that we have ReferenceCountedDisposable, I also take advantage of
that type to change how we do caching: previously, we would keep filling
this cache until the solution close, and then we'd clear out everything
at once. There are two things that aren't great about this pattern:

1. It's a small memory leak, at least up until solution close if you
   stopped using a rule set -- we'll keep holding onto it until we
   close everything.
2. It assumes that once solution close happens, all projects are indeed
   gone. This isn't a good assumption if we ever open up our project
   APIs and let anybody add things to VisualStudioWorkspace, since
   projects may then live past the solution close event.