[SPARK-8014] [SQL] Avoid premature metadata discovery when writing a...

[SPARK-8014] [SQL] Avoid premature metadata discovery when writing a HadoopFsRelation with a save mode other than Append

The current code references the schema of the DataFrame to be written before checking save mode. This triggers expensive metadata discovery prematurely. For save mode other than `Append`, this metadata discovery is useless since we either ignore the result (for `Ignore` and `ErrorIfExists`) or delete existing files (for `Overwrite`) later.

This PR fixes this issue by deferring metadata discovery after save mode checking.

Author: Cheng Lian <lian@databricks.com>

Closes #6583 from liancheng/spark-8014 and squashes the following commits:

1aafabd [Cheng Lian] Updates comments
088abaa [Cheng Lian] Avoids schema merging and partition discovery when data schema and partition schema are defined
8fbd93f [Cheng Lian] Fixes SPARK-8014