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).