Improve performance with many threads (AspNetEnvironment & IISInstanceName)

[304NotModified]

@304NotModified If the usage of ServiceLocator.ServiceProvider should be avoided. Then maybe these should also be fixed:

AspNetEnvironmentLayoutRenderer.cs
IISInstanceNameLayoutRenderer.cs

See #133

and

protected override void CloseLayoutRenderer()
{
   _httpContextAccessor = null;
   base.CloseLayoutRenderer();
}