merge MAPREDUCE-4946 from trunk. Fix a performance problem for large jobs by reducing the number of map completion event type conversions. Contributed by Jason Lowe


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1437104 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'branch-2' of github.com:griddynamics/yhadoop-common into branch-2-gd
Merge branch 'branch-2-gd' of github.com:griddynamics/yhadoop-common into fb-HADOOP-9016-branch-2-gd