[SPARK-5722] [SQL] [PySpark] infer int as LongType

The `int` is 64-bit on 64-bit machine (very common now), we should infer it as LongType for it in Spark SQL.

Also, LongType in SQL will come back as `int`.

Author: Davies Liu <davies@databricks.com>

Closes #4666 from davies/long and squashes the following commits:

6bc6cc4 [Davies Liu] infer int as LongType