Initial websocket request can fail

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I've not been able to pinpoint the reason, but it seems like the initial attempt to connect to a VNCserver via websockets when visiting /desktop can sometimes fail. I suspect its a race condition that may make it never happen in some contexts, and often in others.

In #101 there is now a test that often but not always in our CI system on the first attempt, but not the second.

Debugging

• I've not reproduced this by visiting /desktop on my laptop for tiger or turbo, but I think it does from my desktop computer
• Reproduced using dockerspawner, as seen on the fourth attempt in this GIF:
  

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Ideas on why

1. viewer.js client sends a websocket handshake before websockify is ready to listen
2. websockify receives a websocket handshake before vncserver is ready to listen
3. jupyter-server-proxy's issue WebSocket subprotocols for client/proxy are chosen without asking the server we proxy to jupyter-server-proxy#459 behavior of accepting the client/proxy websocket handshake before the proxy/server handshake.

[consideRatio]

I've seen this show up more consistently when the ping is notable (by testing against a hub deployed in USA from Sweden). Three out of three tests made this happen on first attempt to visit /desktop, while doing things locally I just observed it one out of four for example.