Dial and Accept should return a context that will be cancelled when the connection is closed

[nhooyr]

Just like net/http

[nhooyr]

Not sure what the API should look like. Definitely need to look at the net/http thread in golang-nuts to make sure I do this correctly.

[nhooyr]

This does mean #73 can't remove the read loop as we need it to cancel the context if the client goes away.

[nhooyr]

So the plan is to keep the read goroutine, remove the write goroutine in favour of a goroutine that times the write out.