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PARQUET-1360: Use conforming API style, variable names in WriteFileMetaData functions

Per comments in PARQUET-1348. I also upgraded clang-format to 6.0 here so pardon the diff noise from that.

To summarize the style points:

* Don't pass `const std:shared_ptr<T>&` if `const T&` will do
* Don't pass `const std::shared_ptr<T>&` if `T*` will do
* I prefer to only use class static function members for alternate constructors
* Out arguments after in arguments
* `lower_with_underscores` for variable names

Author: Wes McKinney <wesm+git@apache.org>

Closes #482 from wesm/PARQUET-1360 and squashes the following commits:

375a442 [Wes McKinney] API conformity and changes per review comments in ARROW-1348

Change-Id: I085020ba484b9dd96528c18dad6ff27cb898a0a6