[SPARK-2494] [PySpark] make hash of None consistant cross machines

In CPython, hash of None is different cross machines, it will cause wrong result during shuffle. This PR will fix this.

Author: Davies Liu <davies.liu@gmail.com>

Closes #1371 from davies/hash_of_none and squashes the following commits:

d01745f [Davies Liu] add comments, remove outdated unit tests
5467141 [Davies Liu] disable hijack of hash, use it only for partitionBy()
b7118aa [Davies Liu] use __builtin__ instead of __builtins__
839e417 [Davies Liu] hijack hash to make hash of None consistant cross machines

(cherry picked from commit 872538c600a452ead52638c1ccba90643a9fa41c)
Signed-off-by: Matei Zaharia <matei@databricks.com>