Use TCP Loopback Fast Path on Windows

[benaadams]

As the recommended hosting model is to front Kestrel with a reverse proxy server (e.g. HttpPlatformHandler+IIS) which works over localhost, use the TCP Loopback Fast Path on Windows; where it skips the IP layer.

TCP Loopback Fast Path behaviour reference: http://blogs.technet.com/b/wincat/archive/2012/12/05/fast-tcp-loopback-performance-and-low-latency-with-windows-server-2012-tcp-loopback-fast-path.aspx

The only wrinkle is both end-points need the fast path enabled to receive any advantage - so this may need changes to HttpPlatformHandler if it doesn't already have it enabled.

There appears to be no disadvantage to having it enabled but not used (e.g. other socket not enabled).

When it is used the disadvantage is it loops back further up the stack so fewer packsniffers can listen in; depending on where they insert themselves in the stack. Though localhost is problematic for most of them anyway as they generally listen at the inet level.

Related: "Confirm HttpPlatformHandler uses Fast TCP Loopback" aspnet/IISIntegration#29

[muratg]

@halter73 Please close once the PR is checked in.

[DamianEdwards]

post RC2