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ARROW-4221: [C++][Python] Add canonical flag in COO sparse index

To support the integration with scipy.sparse.coo_matrix, it is necessary to add a flag in SparseCOOIndex.  When this flag is true, a COO sparse tensor has a lexicographically-sorted index and doesn't have duplicated elements.

TODO:
- [X] C++ implementation
- [x] Python implementation
- [x] Add tests

Closes #7477 from mrkn/ARROW-4221

Authored-by: Kenta Murata <mrkn@mrkn.jp>
Signed-off-by: Wes McKinney <wesm@apache.org>