[SPARK-6055] [PySpark] fix incorrect __eq__ of DataType

The _eq_ of DataType is not correct, class cache is not use correctly (created class can not be find by dataType), then it will create lots of classes (saved in _cached_cls), never released.

Also, all same DataType have same hash code, there will be many object in a dict with the same hash code, end with hash attach, it's very slow to access this dict (depends on the implementation of CPython).

This PR also improve the performance of inferSchema (avoid the unnecessary converter of object).

cc pwendell  JoshRosen

Author: Davies Liu <davies@databricks.com>

Closes #4808 from davies/leak and squashes the following commits:

6a322a4 [Davies Liu] tests refactor
3da44fc [Davies Liu] fix __eq__ of Singleton
534ac90 [Davies Liu] add more checks
46999dc [Davies Liu] fix tests
d9ae973 [Davies Liu] fix memory leak in sql