Support AspNetSynchronizationContext

[Bouke]

ASP.NET uses ISyncContextLock for synchronization, which resets the sychronization context after an await.

I've created these changes mostly be trial and error. It seems to work for me, but YMMV.

In ASP.NET MVC 5 try the following:

public ActionResult Index()
{
    TestAsync().Wait();
}

public async Task TestAsync()
{
    Debug.WriteLine($"TestAsync[0]: {SynchronizationContext.Current}");

    await Task.Yield();
    Debug.WriteLine($"TestAsync[1]: {SynchronizationContext.Current}");

    await Task.Delay(1_000).ConfigureAwait(false);
    Debug.WriteLine($"TestAsync[3]: {SynchronizationContext.Current}");
}

In additional testing, I've found some blocking situations that went undetected. Again, YMMV.