Fixed a race condition when attaching to an existing socket

Fixed a race condition when attaching to an existing socket

This is an error that presents itself with RTCDataChannel's, I suspect this could not
happen with a pre-existing WebSocket.

If the remote connection creates a data channel then the local (VNC) side gets a channel
created callback. It may also be the case that in that very same tick the socket is also
opened and buffered data received. This meant that (in my tests) about 1/3 of the time
noVNC would fail to respond to the initial message from the server because it was received
and subsequently not handled during that initial tick.

Also made the documentation reflect this new behaviour and document the existing behaviour.