[SPARK-4479][SQL] Avoids unnecessary defensive copies when sort based shuffle is on

This PR is a workaround for SPARK-4479. Two changes are introduced: when merge sort is bypassed in `ExternalSorter`,

1. also bypass RDD elements buffering as buffering is the reason that `MutableRow` backed row objects must be copied, and
2. avoids defensive copies in `Exchange` operator

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Maybe just use `iterator` in the code below instead of giving it a different name here