Entering passive mode

Let's now write the code to handle the PASV command. Add the following case in handle_cmd():

#[async]
fn handle_cmd(mut self, cmd: Command) -> Result<Self> {
    match cmd {
        // …
        Command::Pasv => self = await!(self.pasv())?,
        // …
    }
}

For the following, we'll need four new fields in the Client structure:

use futures::stream::SplitStream;

use codec::BytesCodec;

type DataReader = SplitStream<Framed<TcpStream, BytesCodec>>;
type DataWriter = SplitSink<Framed<TcpStream, BytesCodec>>;

struct Client {
    data_port: Option<u16>,
    data_reader: Option<DataReader>,
    data_writer: Option<DataWriter>,
    handle: Handle,
    // …
}

And all of them are initialized to None:

impl Client {
    fn new(handle: Handle, writer: Writer, server_root: PathBuf) -> Client {
        Client {
            data_port: None,
            data_reader: None,
            data_writer: None,
            handle,
   ...