[Minor] [SQL] Cleans up DataFrame variable names and toDF() calls

Although we've migrated to the DataFrame API, lots of code still uses `rdd` or `srdd` as local variable names. This PR tries to address these naming inconsistencies and some other minor DataFrame related style issues.

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Author: Cheng Lian <lian@databricks.com>

Closes #4670 from liancheng/df-cleanup and squashes the following commits:

3e14448 [Cheng Lian] Cleans up DataFrame variable names and toDF() calls