DownloadCallback action registered in SFtpClient.DownloadFile can get called after FtpClient is disposed

Currently the callback action is queued on a thread pool thread. This is a problem because you may want to wait for all callback progress actions to complete prior to continuing ... but there is no way of doing that.

I found this in my trace listener that is recording download progress. I won't actually get all events.

These callbacks should either be synchronous, or the download method should complete only once all progress events are done / joined.

The issue is probably also in SFtpClient.Uploadfile

SFtpClient.DownloadFile から始めて、進捗コールバックがスレッドプール上でどのようにキューに入れられるかを調査し、次に SFtpClient.Uploadfile の対応するパスと比較します。trace-listener のシナリオを再現し、すべての進捗コールバックが完了するまでダウンロード操作が戻らないことを検証し、アップロードについても同等の動作を確認します。