[C++] Refactor generic ArrayLoader class, support work for Feather merge

[asfimport]

This will reduce the size of the patch for ARROW-452

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• Add integration tests for date and time types (blocks)

_{Note: This issue was originally created as ARROW-605. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 365
#365