ARROW-6541: [Format][C++] Update Columnar.rst for two-part EOS, update C++ implementation#5361

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wesm wants to merge 3 commits intoapache:ARROW-6313-flatbuffer-alignmentfrom

wesm:two-part-eos

Commits on Sep 11, 2019

Commits on Sep 12, 2019

Copy metadata unconditionally since input buffers may not start on aligned offsets
wesm
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