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ARROW-6065: [C++][Parquet] Clean up parquet/arrow/reader.cc, reduce code duplication, improve readability

This is strictly a refactoring PR. I'm going to start working (for a new PR) on some refactoring of the handling of schemas and nested types (which is also pretty messy in my opinion). The motivation for this is to be able to more cleanly reason about direct dictionary-decoding without having to resort to such hacks as the current `FixSchemas` function

Also cleans up Parquet includes using IWYU

Closes #4963 from wesm/parquet-arrow-read-refactor and squashes the following commits:

cf9f5e303 <Wes McKinney> Add missing API include, fix Python bindings
9d9a30b04 <Wes McKinney> More cleaning
174f046b4 <Wes McKinney> Refactor complete, tests passing again. IWYU pass
02153bf56 <Wes McKinney> More refactoring progress

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