Function send_subscribe 

fn send_subscribe(
    filters: &[(TopicFilterBuf, SubscriptionOptions)],
    subscription_id: Option<u32>,
    queued_tx: Option<Sender<Result<(), Error>>>,
    completion_tx: Sender<Result<Vec<ReasonCode>, Error>>,
    client: &mut Client<JournaledSession>,
    pending: &mut Pending,
    queued_waiting: &mut Vec<Sender<Result<(), Error>>>,
)