[SPARK-9326] Close lock file used for file downloads.

A lock file is used to ensure multiple executors running on the
same machine don't download the same file concurrently. Spark never
closes these lock files (releasing the lock does not close the
underlying file); this commit fixes that.

cc vanzin (looks like you've been involved in various other fixes surrounding these lock files)

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #7650 from kayousterhout/SPARK-9326 and squashes the following commits:

0401bd1 [Kay Ousterhout] Close lock file used for file downloads.