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ARROW-6844: [C++][Parquet] Fix regression in reading List types with item name that is not "item"

This fixes the regression reported in the JIRA. I think this bug got introduced during the various refactorings since 0.14.0.

Closes #5658 from wesm/ARROW-6844 and squashes the following commits:

2065db7fc <Wes McKinney> Rename function with duplicate name
b1965c0cc <Wes McKinney> Use list item names present in Parquet file rather than overriding with 'item'

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>