Reset notified_by list per serialized batch

Fixes the case where a handler attempts to run for every batch after any
batch has notified it, regardless of whether the current batch did.

lib/ansible/playbook/__init__.py

@@ -444,6 +444,7 @@ class PlayBook(object):
                     self.inventory.restrict_to(handler.notified_by)
                     self._run_task(play, handler, True)
                     self.inventory.lift_restriction()
+                    handler.notified_by = []
 
             self.inventory.lift_also_restriction()