HDFS-5698. Use protobuf to serialize / deserialize FSImage. Contributed by Haohui Mai.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1566359 13f79535-47bb-0310-9956-ffa450edef68
Merging r1566042 through r1566364 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-4685@1566375 13f79535-47bb-0310-9956-ffa450edef68