Refactor common getSortField logic in various FieldTypes

  @Override
  public SortField getSortField(SchemaField field, boolean top) {
    field.checkSortability();

    Object missingValue = null;
    boolean sortMissingLast = field.sortMissingLast();
    boolean sortMissingFirst = field.sortMissingFirst();

    if (sortMissingLast) {
      missingValue = top ? SOMECLASS.MIN_VALUE : SOMECLASS.MAX_VALUE;
    } else if (sortMissingFirst) {
      missingValue = top ? SOMECLASS.MAX_VALUE : SOMECLASS.MIN_VALUE;
    }
    SortField sf = new SortField(field.getName(), SortField.Type.SOMETYPE, top);
    sf.setMissingValue(missingValue);
    return sf;
  }

  @Override
  public static SortField getSortField(SchemaField field, boolean top, SortField.Type sortType, 
                                       Object missingLow, Object missingHigh) {

    field.checkSortability();

    Object missingValue = null;
    boolean sortMissingLast = field.sortMissingLast();
    boolean sortMissingFirst = field.sortMissingFirst();

    if (sortMissingLast) {
      missingValue = top ? missingLow : missingHigh;
    } else if (sortMissingFirst) {
      missingValue = top ? missingHigh : missingLow;
    }
    SortField sf = new SortField(field.getName(), sortType, top);
    sf.setMissingValue(missingValue);
    return sf;
  }

  @Override
  public SortField getSortField(SchemaField field, boolean top) {
    return getSortField(field, top, SortField.Type.SOMETIME, 
                        SOMECLASS.MIN_VALUE, SOMECLASS.MAX_VALUE);
  }