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

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1514024 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/trunk@1514068 13f79535-47bb-0310-9956-ffa450edef68