@Override
  public ExtendedCell copyCellInto(ExtendedCell cell) {
    throw new IllegalStateException("This is an Immutable MemStoreLAB.");
  }
A MemStoreLAB implementation which wraps N MemStoreLABs. Its main duty is in proper managing the
close of the individual MemStoreLAB. This is treated as an immutable one and so do not allow to
add any more Cells into it. {@link #copyCellInto(ExtendedCell)} throws Exception
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/ImmutableMemStoreLAB.java