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ARROW-5009: [C++] Remove using std::.* where I could find them

This is more of a proposal to see if people are okay with it, I'm okay closing the PR.  I also am touching parquet files, please let me know if I should open a separate JIRA for those.

We seem to pretty consistently use std:: in most places and sometimes inconsistently within files, this fixes those style issues.

Author: Micah Kornfield <emkornfield@gmail.com>

Closes #4029 from emkornfield/remove_unused_std_string and squashes the following commits:

cfa08c629 <Micah Kornfield> fix missing std
bd4090419 <Micah Kornfield> Remove using std::.* where I could find them