As suggested by James in https://phabricator.services.mozilla.com/D131012#inline-760281 it would be good to have a helper that helps to enter sync mode in the following way:

enter_sync_mode(stream: &mut TcpStream, mode: SyncCommand, path: &UnixPath) -> ... {
        let message = encode_message(&format!("host:transport:{}", self.serial))?;
        stream.write_all(message.as_bytes())?;
        let _bytes = read_response(&mut stream, false, true)?;

        // Send "sync:" command to initialize file transfer
        let message = encode_message("sync:")?;
        stream.write_all(message.as_bytes())?;
        let _bytes = read_response(&mut stream, false, true)?;
}