websocket: closing a connection may block for 5s

[marten-seemann]

See coder/websocket#355 for details. This might block the connection manager loop and the swarm.Close for a long time, if we have a lot of WebSocket connections and this happens multiple times.
Not sure how realistic this is in practice, but I wouldn't want to risk it.

[MarcoPolo]

Could we read the rest of the frame and discard before calling close as a temporary workaround?