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ARROW-3408: [C++] Add CSV option to automatically attempt dict encoding

This is tied to type inference, and only triggers on string or binary columns.
Each chunk is dict-encoded up to a certain cardinality, after which the whole
column falls back on plain encoding.

Closes #5785 from pitrou/ARROW-3408-csv-auto-dict-encode and squashes the following commits:

8d909fdcd <Antoine Pitrou> ARROW-3408:  Add CSV option to automatically attempt dict encoding

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