fix: ChunkedUploader Resume Not Closing ThreadPoolExecutor Instances (#840)

Author: mhagmajer

boxsdk/util/chunked_uploader.py

@@ -153,6 +153,7 @@ def _commit_and_erase_stream_reference_when_succeed(self):
         # Remove file stream reference when uploading file succeeded
         if commit_result is not None:
             self._content_stream = None
+        self._executor.shutdown(wait=True)
         return commit_result