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).
Here I declare `eventlet.wsgi.server()` to have "all requests are finished processing" Happens Before "server() function returned" semantic, no matter what pool was used. Straight and simple.

When/if we need to skip pool synchronization, add option parameter to modify current behavior. And I really doubt it will be ever needed.