void setExtendedCells(ArrayList<ExtendedCell> cells) {
    this.cells = cells;
    this.families = null;
  }
This is not thread safe. This will change the WALEdit and shouldn't be used unless you are sure
that nothing else depends on the contents being immutable.
@param cells the list of cells that this WALEdit now contains.
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/WALEdit.java