[C++] Implement functions to conform unequal dictionaries amongst multiple Arrow arrays

[asfimport]

We may wish to either

• Conform the dictionary indices to reference a common dictionary
• Concatenate indices into a new array with a common dictionary

This is related to in-memory dictionary encoding, as you start with a partially-built dictionary and then add entries as you observe new ones in other dictionaries, all the while "rebasing" indices to consistently reference the same dictionary at the end

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

PRs and other links:

• GitHub Pull Request #3165

_{Note: This issue was originally created as ARROW-554. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Does it make sense to make those a kernel or rather a DictionaryArray method?

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3165
#3165