Refactor DataSource to ensure schema is inferred only once when creating a file stream

When creating a file stream using sqlContext.write.stream(), existing files are scanned twice for finding the schema

• Once, when creating a DataSource + StreamingRelation in the DataFrameReader.stream()
• Again, when creating streaming Source from the DataSource, in DataSource.createSource()

Instead, the schema should be generated only once, at the time of creating the dataframe, and when the streaming source is created, it should just reuse that schema