Fixes#5680.
To make sure we're always reading from the WebSocket connection, we need
to always have an ongoing (but blocking) conn.ReadMessage() call going
on. To achieve this, we do the read in a separate goroutine and write to
a buffered channel. That way we can always read the next message while
the current one is being forwarded. This allows incoming ping messages
to be received and processed which then leads to the deadlines to be
extended correctly.