Begins an asynchronous request to download data from the FTP server and return it as a byte-array.
An IAsyncResult that references the asynchronous download.
You can create a callback method that implements the AsyncCallback delegate and pass its name to the BeginDownloadByteArray method. The state argument may be used to pass in data that will be passed to the callback method.
For more information please refer to the synchronous version of this method, DownloadByteArray.