BplusTree Key's are not generic

[arjunsk]

In ConstantSerializer we are assuming Index as Integer. We need to modify that logic to be generic enough for Constant class.

public class ConstantSerializer implements Serializer<D_Constant> {

  @Override
  public D_Constant read(LargeByteBuffer bb) {
    int ival = bb.getInt();
    return new D_Constant(ival);
  }