send SIGQUIT/SIGTERM only once to the workers

there is no need to send more than once the signal to the workers when
exiting. With this patch we are sending once the QUIT/TERM signal and  then
wait until the graceful timeout or until there are workers alive.

fix #655

gunicorn/arbiter.py

@@ -348,9 +348,12 @@ def stop(self, graceful=True):
         if not graceful:
             sig = signal.SIGQUIT
         limit = time.time() + self.cfg.graceful_timeout
+        # instruct the workers to exit
+        self.kill_workers(sig)
+        # wait until the graceful timeout
         while self.WORKERS and time.time() < limit:
-            self.kill_workers(sig)
             time.sleep(0.1)
+
         self.kill_workers(signal.SIGKILL)
 
     def reexec(self):