in fastpay_core/src/downloader.rs [32:40]
fn query(&mut self, key: Self::Key) -> future::BoxFuture<Self::Value>;
}
/// Channel for using code to send requests and stop the downloader task.
#[derive(Clone)]
pub struct DownloadHandle<K, V>(mpsc::UnboundedSender<DownloadCommand<K, V>>);
/// A command send to the downloader task.
enum DownloadCommand<K, V> {