open_async file is closed on arrival

[vttrifonov]

Not sure if the open_async functionality is ready for release but it returns a file which is closed on arrival.

f = await fs.open_async(path, 'wb')
f.closed --> True

S3AsyncStreamedFile.init never calls it super which takes care of marking the file as "not closed".

[martindurant]

Please do fix; but you are right that the async files are not done and perhaps should be abandoned. GenericFileSystem does use them, though, since that involves moving bytes between filesystems, so streaming is important.