HDFS-4464. Combine collectSubtreeBlocksAndClear with deleteDiffsForSnapshot and rename it to destroySubtreeAndCollectBlocks.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1441680 13f79535-47bb-0310-9956-ffa450edef68