[C++] Investigate if the *_data_ pointers used in Builder classes improve performance on hot paths

[asfimport]

See @alendit comment in #2315 (comment)

Reporter: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #3863

_{Note: This issue was originally created as ARROW-2905. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
It ain't broke so no need to fix right now