* use tornado streaming for uploads

use tornado.web.stream_request_body for the upload handler to avoid
reading the entire upload into a memory buffer *in the IOLoop*. Instead,
hash the content as it's streamed in and store it in a temporary file under
COURSELABS. Then the upload business logic can just rename the temporary file
over the destination if the upload is accepted. This significantly improves
latency when large files are uploaded

* Remove tempfile if it's a duplicate

Remove the temporary upload file if it's a duplicate instead of leaving it
in the temp directory.