ARROW-6729: [C++] Prevent data copying in StlStringBuffer
ARROW-6729: [C++] Prevent data copying in StlStringBuffer

Closes #5517 from st-pasha/stl-buffer-nocopy and squashes the following commits:

6f26b4dea <Pasha Stetsenko> try to satisfy lint
2d4dda6c4 <Pasha Stetsenko> remove whitespace
85e91f5e4 <Pasha Stetsenko> Prevent data copying in StlStringBuffer

Authored-by: Pasha Stetsenko <7677ca593fffbc1f3ecbe757ec62779e2e67903c@h2o.ai>
Signed-off-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>

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