[SPARK-17240][CORE] Make SparkConf serializable again.

Make the config reader transient, and initialize it lazily so that
serialization works with both java and kryo (and hopefully any other
custom serializer).

Added unit test to make sure SparkConf remains serializable and the
reader works with both built-in serializers.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #14813 from vanzin/SPARK-17240.