Fix a race condition when releasing a Python worker

There is a race condition when releasing a Python worker. If "ReaderIterator.handleEndOfDataSection" is not running in the task thread, when a task is early terminated (such as "take(N)"), the task completion listener may close the worker but "handleEndOfDataSection" can still put the worker into the worker pool to reuse.

https://github.com/zsxwing/spark/commit/0e07b483d2e7c68f3b5c3c118d0bf58c501041b7 is a patch to reproduce this issue.

I also found a user reported this in the mail list: http://mail-archives.apache.org/mod_mbox/spark-user/201610.mbox/%3CCAAUq=H+YLUEpd23nwvq13Ms5hOStkhX3ao4f4zQV6sgO5zM-xA@mail.gmail.com%3E