Consider enforcing ALPN negotiation

[Lukasa]

Right now both the server and the client ignore the result of the ALPN negotiation. The client does log the outcome, but otherwise does nothing: the server largely ignores it and sniffs the initial content. This is likely suboptimal: the ALPN is normative and we should probably pay attention to it. In some cases we likely want to skip that, but we can make that configuration.