Fix eventlet shutdown to actively shut down the workers.

This is a simple commit of the patch the @tilgovi placed on issue #867.

@tilgovi I took the liberty of attributing you as the author of the commit.
I'm worried about the use of `_stop_checker` here, because it's probably not supposed to be public API. I had just copied this code out of some part of eventlet. I wonder if we need it. If we do, I wonder how portable it is across eventlet versions.

Any ideas or any possibility you can look into it?