IMPALA-639: Fix per node peak mem usage reporting.

Change-Id: Ie130df4f9434ae8d9ff500afc41a17010927338a
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3729
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3737