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ARROW-6789: [Python] Improve ergonomics by automatically boxing Action and Result in do_action RPC

This allows clients to invoke actions like

```
client.do_action('bodyless-action-name')
client.do_action(('action-with-body', b'action-body'))
```

On the server side, the results of `do_action` can be `str`, `bytes`, or `pyarrow.Buffer` and will be automatically converted to `Result`.

Closes #5651 from wesm/ARROW-6789 and squashes the following commits:

b5316c315 <Wes McKinney> Fix Python 2.7
47d0aa5a7 <Wes McKinney> Improve Buffer.__eq__
bccc6bd69 <Wes McKinney> Code review feedback
53d2c87cd <Wes McKinney> Automatically box Action and Result in do_action RPC

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>