arrow

ARROW-1402: [C++] Deprecate APIs which return std::shared_ptr<MutableBuffer> in favor of std::shared_ptr<Buffer>

My view is that exposing this detail serves only to complicate the user API, since instances of the base class are permitted to be mutable (when `buffer->mutable_data()` is not null).

These APIs are used in parquet-cpp, but the build here should pass if the deprecations are handled properly. I will submit a patch there to fix these -- that patch can go in after parquet-cpp 1.3.0 to avoid any disruptions (so parquet-cpp will work fine with either Arrow 0.6.0 or 0.7.0).

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

Closes #1001 from wesm/ARROW-1402 and squashes the following commits:

c8caae7 [Wes McKinney] Deprecate APIs which return std::shared_ptr<MutableBuffer> in favor of std::shared_ptr<Buffer>