@Override
  public ExtendedCell next() throws IOException {
    if (closed) {
      return null;
    }
    ExtendedCell oldCurrent = current;
    updateCurrent(); // update the currently observed Cell
    return oldCurrent;
  }
Return the next Cell in this scanner, iterating the scanner
@return the next Cell or null if end of scanner
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/SegmentScanner.java