Formerly we waiting until *both* directions finished. What this meant in
practice is that when the remote connection ended, copyLoop would become
useless but would continue blocking its caller until something else
finally closed the socks connection.
The call was
copyLoop(socks, snowflake)
but the function signature was
func copyLoop(WebRTC, SOCKS io.ReadWriter) {
The mistake was mostly harmless, because both arguments were treated the
same, except that error logs would have reported the wrong direction.
