[SPARK-5581][CORE] When writing sorted map output file, avoid open / …

…close between each partition

## What changes were proposed in this pull request?

Replace commitAndClose with separate commit and close to avoid opening and closing
the file between partitions.

## How was this patch tested?

Run existing unit tests, add a few unit tests regarding reverts.

Observed a ~20% reduction in total time in tasks on stages with shuffle
writes to many partitions.

JoshRosen

Author: Brian Cho <bcho@fb.com>

Closes #13382 from dafrista/separatecommit-master.