Fixed #16759 -- Remove use of __deepcopy__ in qs.clone()

The original problem was that queryset cloning was really expensive
when filtering with F() clauses. The __deepcopy__ went too deep copying
_meta attributes of the models used. To fix this the use of
__deepcopy__ in qs cloning was removed.

This commit results in some speed improvements across the djangobench
benchmark suite. Most query_* tests are 20-30% faster, save() is 50%
faster and finally complex filtering situations can see 2x to order
of magnitude improvments.

Thanks to Suor, Alex and lrekucki for valuable feedback.

tests/regressiontests/queries/tests.py

@@ -1919,6 +1919,7 @@ class SubqueryTests(TestCase):
 
 
 class CloneTests(TestCase):
+
     def test_evaluated_queryset_as_argument(self):
         "#13227 -- If a queryset is already evaluated, it can still be used as a query arg"
         n = Note(note='Test1', misc='misc')
@@ -1933,6 +1934,39 @@ class CloneTests(TestCase):
         # that query in a way that involves cloning.
         self.assertEqual(ExtraInfo.objects.filter(note__in=n_list)[0].info, 'good')
 
+    def test_no_model_options_cloning(self):
+        """
+        Test that cloning a queryset does not get out of hand. While complete
+        testing is impossible, this is a sanity check against invalid use of
+        deepcopy. refs #16759.
+        """
+        opts_class = type(Note._meta)
+        note_deepcopy = getattr(opts_class, "__deepcopy__", None)
+        opts_class.__deepcopy__ = lambda obj, memo: self.fail("Model options shouldn't be cloned.")
+        try:
+            Note.objects.filter(pk__lte=F('pk') + 1).all()
+        finally:
+            if note_deepcopy is None:
+                delattr(opts_class, "__deepcopy__")
+            else:
+                opts_class.__deepcopy__ = note_deepcopy
+
+    def test_no_fields_cloning(self):
+        """
+        Test that cloning a queryset does not get out of hand. While complete
+        testing is impossible, this is a sanity check against invalid use of
+        deepcopy. refs #16759.
+        """
+        opts_class = type(Note._meta.get_field_by_name("misc")[0])
+        note_deepcopy = getattr(opts_class, "__deepcopy__", None)
+        opts_class.__deepcopy__ = lambda obj, memo: self.fail("Model fields shouldn't be cloned")
+        try:
+            Note.objects.filter(note=F('misc')).all()
+        finally:
+            if note_deepcopy is None:
+                delattr(opts_class, "__deepcopy__")
+            else:
+                opts_class.__deepcopy__ = note_deepcopy
 
 class EmptyQuerySetTests(TestCase):
     def test_emptyqueryset_values(self):