HDFS-5219. Add configuration keys for retry policy in WebHDFSFileSystem. Contributed by Haohui Mai.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1524498 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5031. BlockScanner scans the block multiple times. (Vinay via Arpit Agarwal)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1524553 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5122. Support failover and retry in WebHdfsFileSystem for NN HA. Contributed by Haohui Mai.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1524562 13f79535-47bb-0310-9956-ffa450edef68