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If we send no message, the client gets a 1005. This will send a 1000.

I believe this was written before request hijacking was a thing. We no longer
need to hold the `ServeHTTP` function open.

(make @whyrusleeping happy)

1. Ensure we only close the connection once. Especially, don't call the done
function multiple times and/or concurrently.
2. Call WriteControl instead of WriteMessage in Close. WriteControl is
thread-safe, WriteMessage isn't.

Also, this sets a 100ms deadline on gracefully closing connections.