Lock the Scheduler model for the database it's using

This can happen when you use a database router for example

django_q/cluster.py

@@ -546,7 +546,7 @@ def scheduler(broker=None):
         broker = get_broker()
     close_old_django_connections()
     try:
-        with db.transaction.atomic():
+        with db.transaction.atomic(using=Schedule.objects.db):
             for s in (
                 Schedule.objects.select_for_update()
                 .exclude(repeats=0)