SPARK-1432: Make sure that all metadata fields are properly cleaned

While working on spark-1337 with @pwendell, we noticed that not all of the metadata maps in JobProgessListener were being properly cleaned. This could lead to a (hypothetical) memory leak issue should a job run long enough. This patch aims to address the issue.

Author: Davis Shepherd <davis@conviva.com>

Closes #338 from dgshep/master and squashes the following commits:

a77b65c [Davis Shepherd] In the contex of SPARK-1337: Make sure that all metadata fields are properly cleaned
(cherry picked from commit a3c51c6ea2320efdeb2a6a5c1cd11d714f8994aa)

Conflicts:

	core/src/main/scala/org/apache/spark/ui/jobs/JobProgressListener.scala