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.tests/test/org/eclipse/birt/data/engine/api/APITestCase.java
queryDefn.setDataSetName( this.dataSet.getName( ) );
		else
			queryDefn.setDataSetName( dataset.getName( ) );
		
		return queryDefn;
	}
	
	/**
	 * Execute query definition
	 * 
	 * @param query
	 * @return resultIterator
	 * @throws Exception
	 */
	protected IResultIterator executeQuery( IQueryDefinition query )
			throws Exception
	{
		IPreparedQuery preparedQuery = dataEngine.prepare( query );
		IQueryResults queryResults = preparedQuery.execute( null );
		return queryResults.getResultIterator( );
	}
	
	/**
	 * Output result of executing query defintion
	 * 
	 * @param resultIt
	 * @param expressions
	 * @throws Exception
	 */
	protected void outputQueryResult( IResultIterator resultIt,