@Override
          public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs)
              throws IOException {
            return FileVisitResult.CONTINUE;
          }
Search all data files in one directory recursively.

@return
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/snapshot/SnapshotLoader.java