Bug 213126 Add support for DataEngine-leve dataset cache
In many cases the DataEngine-leve dataset cache is useful which ensure that a dataset is executed one time during rendering one report. To support this feature a new dataset property should be added. The Birt customer can choose whether this dataset need to be cached or not by setting the property in deisgn time.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/DataSetHandle.java
{
		PropertyHandle propHandle = getPropertyHandle( RESULT_SET_HINTS_PROP );
		assert propHandle != null;
		return propHandle.iterator( );
	}
	/**
	 * Returns an iterator for the list of computed columns. The iterator
	 * returns instances of <code>ComputedColumnHandle</code> that represents
	 * computed column object.
	 * 
	 * @return iterator over computed columns.
	 * @see org.eclipse.birt.report.model.api.elements.structures.ComputedColumn
	 */
	public Iterator computedColumnsIterator( )
	{
		PropertyHandle propHandle = getPropertyHandle( IDataSetModel.COMPUTED_COLUMNS_PROP );
		assert propHandle != null;
		return propHandle.iterator( );
	}
	/**
	 * Returns an iterator over column hints. The iterator returns instances of
	 * <code>ColumnHintHandle</code> that represents column hint object.
	 * 
	 * @return iterator over column hints.
	 * 
	 * @see org.eclipse.birt.report.model.api.elements.structures.ColumnHint
	 */
	public Iterator columnHintsIterator( )
	{
		PropertyHandle propHandle = getPropertyHandle( IDataSetModel.COLUMN_HINTS_PROP );