Add HDFS-7923 to CHANGES.txt
Add HDFS-7923 to CHANGES.txt

zjshen/hadoop-YARN-2928-POC,zjshen/hadoop-YARN-2928-POC,zjshen/hadoop-YARN-2928-POC,zjshen/hadoop-YARN-2928-POC,zjshen/hadoop-YARN-2928-POC,zjshen/hadoop-YARN-2928-POC,zjshen/hadoop-YARN-2928-POC