[SPARK-19092][SQL][BACKPORT-2.1] Save() API of DataFrameWriter should not scan all the saved files #16481

### What changes were proposed in this pull request?

#### This PR is to backport https://github.com/apache/spark/pull/16481 to Spark 2.1
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`DataFrameWriter`'s [save() API](https://github.com/gatorsmile/spark/blob/5d38f09f47a767a342a0a8219c63efa2943b5d1f/sql/core/src/main/scala/org/apache/spark/sql/DataFrameWriter.scala#L207) is performing a unnecessary full filesystem scan for the saved files. The save() API is the most basic/core API in `DataFrameWriter`. We should avoid it.

### How was this patch tested?
Added and modified the test cases

Author: gatorsmile <gatorsmile@gmail.com>

Closes #16588 from gatorsmile/backport-19092.