[1.8.x] Fixed #24381 -- removed ForeignObjectRel opts and to_opts

These cached properies were causing problems with pickling, and in
addition they were confusingly defined: field.rel.model._meta was
not the same as field.rel.opts.

Instead users should use field.rel.related_model._meta inplace of
field.rel.opts, and field.rel.to._meta in place of field.rel.to_opts.

Backport of f95122e541 from master

tests/queryset_pickle/tests.py

@@ -43,6 +43,9 @@ class PickleabilityTestCase(TestCase):
     def test_membermethod_as_default(self):
         self.assert_pickles(Happening.objects.filter(number4=1))
 
+    def test_filter_reverse_fk(self):
+        self.assert_pickles(Group.objects.filter(event=1))
+
     def test_doesnotexist_exception(self):
         # Ticket #17776
         original = Event.DoesNotExist("Doesn't exist")