HADOOP 9871. Fix intermittent findbugs warnings in DefaultMetricsSystem. (Junping Du via llu)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1514025 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-9845. Update protobuf to 2.5 from 2.4.x. (tucu)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1514071 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-9872. Improve protoc version handling and detection. (tucu)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1514074 13f79535-47bb-0310-9956-ffa450edef68