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.
That is what bothered me and why I didn't immediately issue a PR. I didn't want to copy/paste code from eventlet core that uses non-public APIs, but I didn't see any way to get at the underlying pool without it. I'm not familiar with eventlet, so I don't know if the link to the stop checker is necessary or if there's another approach that's more sane.