Uniqueness assert of section controllers, close #567

Summary:
Issue fixed: #567

- [ ] All tests pass. Demo project builds and runs.
- [ ] I added tests, an experiment, or detailed why my change isn't tested.
- [ ] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/639

Differential Revision: D4878842

Pulled By: jessesquires

fbshipit-source-id: 664648cf4f5e28d0a6af308e30151cb66863962c

Source/IGListAdapter.m

@@ -529,6 +529,11 @@
         }
 
         [sectionControllers addObject:sectionController];
+
+#if DEBUG
+        IGAssert([NSSet setWithArray:sectionControllers].count == sectionControllers.count,
+                 @"Section controllers array is not filled with unique objects; section controllers are being reused");
+#endif
         [validObjects addObject:object];
     }