[arbiter] close sockets on shutdown

Close all the listeners when the arbiter shuts down. By doing so,
workers can close the socket at the beginning of a graceful shut
down thereby informing the operating system that the socket can
be cleaned up. With this change, graceful exits with such workers
will refuse new connections while draining, allowing load balancers
to respond more quickly and avoiding leaving connections dangling
in the listen backlog, unaccepted.

Ref #922

gunicorn/arbiter.py

@@ -334,6 +334,8 @@ def stop(self, graceful=True):
         :attr graceful: boolean, If True (the default) workers will be
         killed gracefully  (ie. trying to wait for the current connection)
         """
+        for l in self.LISTENERS:
+            l.close()
         self.LISTENERS = []
         sig = signal.SIGTERM
         if not graceful: