Properly implement Close Handshake

[nhooyr]

See #103

So the problem here is not really that the close handshake is useful but that a lot of libraries expect to be able to reply to a close frame with a echoed close frame and if the connection is closed before they can reply, they error out and it is confusing.

So I'm going to revive my effort in #104 to read the connection automatically in Close() and discard all messages until a Close frame is received for 5s.

This should also go into the README as a nice behavioural advantage over gorilla/websocket and gobwas/ws.

[nhooyr]

See https://circleci.com/gh/nhooyr/websocket/16?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-checks-link for a test failure that reproduces this.

That test expected an error that indicates the remote WebSocket status frame but instead it got a write failure error out because the remote end closed the connection right after sending the close frame. This is also reproducible with gorilla/websocket and I'm sure a host of other libraries.

[nhooyr]

My WIP branch is at https://github.com/nhooyr/websocket/tree/close-handshake

[nhooyr]

Nvm, can't reproduce with gorilla/websocket as it returns the CloseError regardless of whether or not the close frame write succeeded. Maybe we ought to do this too as there is no issue on gorilla/websocket about this and it is much simpler a solution.

[nhooyr]

Even if gorilla/websocket gets it right, other WebSocket libraries might not so I will need to survey them.

[nhooyr]

I say we just keep it simple for now, if it ends up being an issue, I'll keep that branch so we can refer to it and easily implement it.