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

Closes #5361 from wesm/two-part-eos and squashes the following commits:

cd632ffcb <Wes McKinney> Copy metadata unconditionally since input buffers may not start on aligned offsets
70d1b3ae3 <Wes McKinney> Remove unneeded header
407152855 <Wes McKinney> Use 2-part EOS in C++, update in format docs. Write old EOS if option is set

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