C++: Implement DictionaryBuilder

[asfimport]

Now that we have a hash table inside of Arrow, I would start to implement a DictionaryBuilder for primitive types based on the AdaptiveIntBuilder.

Reporter: Uwe Korn / @xhochy
Assignee: Uwe Korn / @xhochy

_{Note: This issue was originally created as ARROW-1160. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
I didn't move the dictionary encoder from parquet/encoding.h but it should be easy enough to adapt (didn't make sense to move to Arrow because it run-length encodes the indices)

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 792
#792