[C++] Add an incomplete dictionary type

[asfimport]

This would allow to pass a DataType that means "dict-encoded data with the given index types and value types, but the actual values are not yet known" (they might be inferred by processing non-dict-encoded data, or they might be transferred explicitly - but later - in the data stream).

Reporter: Antoine Pitrou / @pitrou

Related issues:

• [C++] Add option to CSV reader to dictionary encode individual columns or all string / binary columns (is depended upon by)
• [C++] Add support for reading Flight streams with dictionaries (is depended upon by)

PRs and other links:

• GitHub Pull Request #4067

_{Note: This issue was originally created as ARROW-5052. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Closing in favor of solution merged in ARROW-3144