[SPARK-7810] [PYSPARK] solve python rdd socket connection problem

Method "_load_from_socket" in rdd.py cannot load data from jvm socket when ipv6 is used. The current method only works well with ipv4. New modification should work around both two protocols.

Author: Ai He <ai.he@ussuning.com>
Author: AiHe <ai.he@ussuning.com>

Closes #6338 from AiHe/pyspark-networking-issue and squashes the following commits:

d4fc9c4 [Ai He] handle code review 2
e75c5c8 [Ai He] handle code review
5644953 [AiHe] solve python rdd socket connection problem to jvm