Bug 180842 Config memory buffer size in data engine execution.
To tuning the performance, it is necessary for DtE to expose API for clients to config the size of memory it used to store the final result set. A new AppContext string, "org.eclipse.birt.data.cache.memory.size" is proposed. The new appcontext entry only accept positive integer as input, the unit of which would be MB.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/impl/QueryExecutor.java
* 
	 * @param cx
	 * @param dataSetFilters
	 * @param queryFilters
	 * @param temporaryComputedColumns
	 * @return
	 * @throws DataException 
	 */
	private List prepareFilters( Context cx, List dataSetFilters,
			List queryFilters, List temporaryComputedColumns ) throws DataException
	{
		List result = new ArrayList( );
		prepareFilter( cx, dataSetFilters, temporaryComputedColumns, result );
		prepareFilter( cx, queryFilters, temporaryComputedColumns, result );
		return result;
	}
	/**
	 * 
	 * @param cx
	 * @param dataSetFilters
	 * @param temporaryComputedColumns
	 * @param result
	 * @throws DataException 
	 */
	private void prepareFilter( Context cx, List dataSetFilters,
			List temporaryComputedColumns, List result ) throws DataException
	{
		if ( dataSetFilters != null && !dataSetFilters.isEmpty( ) )
		{
			Iterator it = dataSetFilters.iterator( );