Make _Scheduler that has been shutdown ignore schedule requests

There is no reason to accept schedule requests when cluster is shutting
down.

Author: dkropachev

cassandra/cluster.py

@@ -4456,6 +4456,8 @@ def shutdown(self):
         self.join()
 
     def schedule(self, delay, fn, *args, **kwargs):
+        if self.is_shutdown:
+            return
         if delay:
             self._insert_task(delay, (fn, args, tuple(kwargs.items())))
         else: