[C++] Clean up arrow:: public API

[asfimport]

There are a lot of internal classes, types, and functions in the public API docs

http://arrow.apache.org/docs/cpp/namespacearrow.html

it would be more helpful to move some of these to internal:: namespace or otherwise hide them from doxygen

Reporter: Wes McKinney / @wesm
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #2633

_{Note: This issue was originally created as ARROW-3136. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 2633
#2633