Allow more gracefull shutdown of wsgi server

Currently, if the listening socket is closed the
wsgi.server will go into an infinite loop.

Add a 'server_running' flag which can be set to
tell the server to exit. This allows, for example,
a signal handler to initiate a gracefull shutdown
(complete existing requests then exit).
Ok, sounds fair. Thanks.

I'm testing an updated patch, will push it up soon.

Thanks for your help.