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ARROW-455: [C++] Add dtor to BufferOutputStream that calls Close()

Since `Close()` can technically fail, it's better to call it yourself (and it's idempotent), but this will help avoid a common class of bugs in small-scale use cases.

An alternative here is that we could remove all `Close()` calls from all destructors and possibly add a `DCHECK(!is_open_)` to the base dtor to force the user to close handles. The downside of this is that it makes RAII more difficult, so I'd prefer to leave the close-in-dtor even though it can fail in unusual scenarios.

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #269 from wesm/ARROW-455 and squashes the following commits:

821ee22 [Wes McKinney] Add dtor to BufferOutputStream that calls Close()