Function write_message 

async fn write_message<W, M>(writer: W, msg: Option<&M>) -> Result<()>
where
    W: AsyncWrite + Unpin,
    M: Message,

Write a message into the given writer.

The message can be None, in which case an empty message is written. This is used to implement keepalives. At the receiver, empty messages are ignored, but they do reset the read timeout.