[SPARK-7902] [SPARK-6289] [SPARK-8685] [SQL] [PYSPARK] Refactor of serialization for Python DataFrame

This PR fix the long standing issue of serialization between Python RDD and DataFrame, it change to using a customized Pickler for InternalRow to enable customized unpickling (type conversion, especially for UDT), now we can support UDT for UDF, cc mengxr .

There is no generated `Row` anymore.

Author: Davies Liu <davies@databricks.com>

Closes #7301 from davies/sql_ser and squashes the following commits:

81bef71 [Davies Liu] address comments
e9217bd [Davies Liu] add regression tests
db34167 [Davies Liu] Refactor of serialization for Python DataFrame