HDFS-5010. Reduce the frequency of getCurrentUser() calls from namenode. Contributed by Kihwal Lee.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1505160 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-xxx-hadoop-auth-kerberos-realm-trunk-gd