Improved overall consistency.
- Moved old aggregate tests to a deprecated folder. New tests will be
  written soon.
- Changed all nodes to handle {tuples, Tuples} messages only (instead of
  both {tuple, Tuple} and {tuples, Tuples}
- Used send_tuples and send_diffs in appropriate places.

- Fixed tests as a result of these changes.
- Minor changes to comments here and there.