offloaded: close reader before removing its file

unlinking a file before closing it is not portable. it works on unix, but fails
since an open file holds a lock on windows.

this closes the reader before trying to unlink the encoding file within the 
CHKUploadHelper.

src/allmydata/offloaded.py

@@ -92,6 +92,7 @@ class CHKUploadHelper(Referenceable, upload.CHKUploader):
     def _finished(self, res):
         (uri_extension_hash, needed_shares, total_shares, size) = res
         upload_results = {'uri_extension_hash': uri_extension_hash}
+        self._reader.close()
         os.unlink(self._encoding_file)
         self._finished_observers.fire(upload_results)
         self._helper.upload_finished(self._storage_index)