Distcp with snapshot diff to support Ozone filesystem.

// currently we require both the source and the target file system are
// DistributedFileSystem or (S)WebHdfsFileSystem.
if (!(srcFs instanceof DistributedFileSystem
        || srcFs instanceof WebHdfsFileSystem)) {
  throw new IllegalArgumentException("Unsupported source file system: "
      + srcFs.getScheme() + "://. " +
      "Supported file systems: hdfs://, webhdfs:// and swebhdfs://.");
}
if (!(tgtFs instanceof DistributedFileSystem
    || tgtFs instanceof WebHdfsFileSystem)) {
  throw new IllegalArgumentException("Unsupported target file system: "
      + tgtFs.getScheme() + "://. " +
      "Supported file systems: hdfs://, webhdfs:// and swebhdfs://.");
}