Separate HttpApplication feature insertion from invocation #417
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This allows the features for HttpApplication to be insterted in the HttpContext at the beginning of the request, but will only be invoked in the
.UseSystemWebAdapters()
middleware call. This enables a request to potentially modify behavior of the features if needed before invoking the middleware.A nice side effect of this is that an HttpApplication instance will only be retrieved from the pool if it is actually used within a request, otherwise, it will never be retrieved. This is one of the more expensive parts of the emulated IIS pipeline as we have to assign an HttpApplication to a request, so it can potentially prevent a request from needing to perform this.