ARROW-1246: [Format] Draft Flatbuffer metadata description for Map
ARROW-1246: [Format] Draft Flatbuffer metadata description for Map

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

Closes #876 from wesm/ARROW-1246 and squashes the following commits:

98790dfc [Wes McKinney] Review feedback to clarify nullability of map components
346b48dd [Wes McKinney] Typo
06ae8ebf [Wes McKinney] Draft Flatbuffer metadata for Map

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