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ARROW-3324: [Python] Destroy temporary metadata builder classes more eagerly when building files to reduce memory usage

Destroy RowGroupMetadataBuilder after each row group is completed

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

Closes #3261 from tanyaschlusser/ARROW-3324 and squashes the following commits:

5f3876706 <Wes McKinney> Refine case a bit
4f2bdcdce <Wes McKinney> Destroy RowGroupMetadataBuilder object after completing a row group to reduce memory usage