[C++] Implement MapType, MapArray, MapBuilder

[asfimport]

Add implementation of MapType https://github.com/apache/arrow/blob/master/format/Schema.fbs#L55-L82

Reporter: Ben Kietzman / @bkietz
Assignee: Ben Kietzman / @bkietz

Related issues:

• [C++] Implement Map logical type (is duplicated by)

_{Note: This issue was originally created as ARROW-5326. Please see the migration documentation for further details.}

[asfimport]

Bryan Cutler / @BryanCutler:
I think this already exists as ARROW-1207 but doesn't mention the builder. Not sure if there should be a separate issue for that.