SPARK-1700: Close socket file descriptors on task completion

This will ensure that sockets do not build up over the course of a job, and that cancellation successfully cleans up sockets.

Tested in standalone mode. More file descriptors spawn than expected (around 1000ish rather than the expected 8ish) but they do not pile up between runs, or as high as before (where they went up to around 5k).

Author: Aaron Davidson <aaron@databricks.com>

Closes #623 from aarondav/pyspark2 and squashes the following commits:

0ca13bb [Aaron Davidson] SPARK-1700: Close socket file descriptors on task completion

(cherry picked from commit 0a14421765b672305e8f32ded4a9a1f6f7241d8d)
Signed-off-by: Aaron Davidson <aaron@databricks.com>
Update SchemaRDD.scala

Modify spelling errors

Author: ArcherShao <ArcherShao@users.noreply.github.com>

Closes #619 from ArcherShao/patch-1 and squashes the following commits:

2957195 [ArcherShao] Update SchemaRDD.scala
(cherry picked from commit 9347565f4188cf1574c6dc49fcde91eb286be955)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>