Add support for new COMPOSITE_CRC in WebHDFS

As a followup to https://issues.apache.org/jira/browse/HDFS-13056, plumbing through support for new FileChecksum types (in particular, COMPOSITE_CRC) in WebHDFS will help remove dependencies on the old DFSClient.getFileChecksum method that hard-codes MD5MD5CRC32FileChecksum as its return type. Once this is done, this work should also include marking the old getFileChecksum method as deprecated or removing it entirely from DFSClient.