If a PeriodicCallback runs longer than its period, skip the missed callbacks

instead of trying to run all the missed callbacks to catch up.

tornado/ioloop.py

@@ -461,7 +461,9 @@ class PeriodicCallback(object):
 
     def _schedule_next(self):
         if self._running:
-            self._next_timeout += self.callback_time / 1000.0
+            current_time = time.time()
+            while self._next_timeout < current_time:
+                self._next_timeout += self.callback_time / 1000.0
             self.io_loop.add_timeout(self._next_timeout, self._run)