HDFS-6863. Archival Storage: Support migration for snapshot paths. Contributed by Jing Zhao.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-6584@1619627 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6906. Archival Storage: Add more tests for BlockStoragePolicy. Contributed by Tsz Wo Nicholas Sze.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-6584@1619628 13f79535-47bb-0310-9956-ffa450edef68