Function send_subscribe 

async fn send_subscribe<W: AsyncWrite + Unpin>(
    filters: Vec<(TopicFilterBuf, SubscriptionOptions)>,
    subscription_id: Option<u32>,
    result_tx: Sender<Result<Vec<ReasonCode>, Error>>,
    client: &mut Client<JournaledSession>,
    writer: &mut W,
    pending: &mut Pending,
) -> Result<(), Error>