[SPARK-3749] [PySpark] fix bugs in broadcast large closure of RDD

1. broadcast is triggle unexpected
2. fd is leaked in JVM (also leak in parallelize())
3. broadcast is not unpersisted in JVM after RDD is not be used any more.

cc JoshRosen , sorry for these stupid bugs.

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

Closes #2603 from davies/fix_broadcast and squashes the following commits:

080a743 [Davies Liu] fix bugs in broadcast large closure of RDD
SPARK-2626 [DOCS] Stop SparkContext in all examples

Call SparkContext.stop() in all examples (and touch up minor nearby code style issues while at it)

Author: Sean Owen <sowen@cloudera.com>

Closes #2575 from srowen/SPARK-2626 and squashes the following commits:

5b2baae [Sean Owen] Call SparkContext.stop() in all examples (and touch up minor nearby code style issues while at it)