Make the code in ListenerSecondary less intense

[lodejard]

It's very hard to verify correctness, and it's likely unmaintainable

See #145

[benaadams]

Resolved by #320 which has been merged?

e.g. https://github.com/aspnet/KestrelHttpServer/blob/dev/src/Microsoft.AspNet.Server.Kestrel/Http/ListenerSecondary.cs

[muratg]

👍