Fixing notify credentials job

To save the memory get cert and grade object directly from
the db because by doing that we'll override the queryset
evaluation and object won't be cached once it has gone out
of scope.

PROD-987

openedx/core/djangoapps/credentials/management/commands/notify_credentials.py

@@ -64,9 +64,10 @@ def paged_query(queryset, delay, page_size):
 
         if delay and page:
             time.sleep(delay)
-
-        for i, item in enumerate(subquery, start=1):
-            yield page_start + i, item
+        index = 0
+        for item in subquery.iterator():
+            index += 1
+            yield page_start + index, item
 
 
 class Command(BaseCommand):