Http Throttling graceful shutdown

When http throttling is enabled, we'll be queueing requests up to the limit set by the **maxOutstandingRequests** limit. In the case of a complete process shutdown (i.e. WebScriptHostManager.Shutdown) we should handle any queued requests gracefully. E.g. any requests that are still sitting in the queue, we should return a 503.

We need to experiment with this. It might be that ASP.NET already gives us correct default behavior and we don't need to do anything.