Simplify HTTPS listening, leverage net/http methods

[artyom]

Make use of net/http Server.ListenAndServeTLS — this gives upcoming
http2 support in Go 1.6 for free.

[jehiah]

@artyom can you clarify for me why Server.Serve() wouldn't support http2 automatically?

[artyom]

At least current http.Server implementation calls private method setupHTTP2 which can only be applied in a meaningful way when tls.Listener is set up by Server.ListenAndServeTLS, since setupHTTP2 calls http2ConfigureServer performing a lot of low-level setup on Server.TLSConfig which is ignored if Server is started on pre-established TLS connection.

[jehiah]

Thanks for the pointers. I've not familiarized myself with the hooks/implications w/r/t http2 as that has started to land in golang tip.

I'll try to familiarize myself w/ the implications of http2 support in net/http and check back in here soon.