[SPARK-6677] [SQL] [PySpark] fix cached classes

It's possible to have two DataType object with same id (memory address) at different time, we should check the cached classes to verify that it's generated by given datatype.

This PR also change `__FIELDS__` and `__DATATYPE__` to lower case to match Python code style.

Author: Davies Liu <davies@databricks.com>

Closes #5445 from davies/fix_type_cache and squashes the following commits:

63b3238 [Davies Liu] typo
47bdede [Davies Liu] fix cached classes