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
dataEngine = DataEngine.newDataEngine( DataEngineContext.newInstance( DataEngineContext.DIRECT_PRESENTATION,
				jsScope,
				null,
				null ) );
		prepareDataSource( );
	}
	
	/*
	 * @see TestCase#tearDown()
	 */
	protected void tearDown( ) throws Exception
	{
		dataEngine.shutdown( );
		closeDataSource( );
		
		super.tearDown( );
	}
	
	/**
	 * Prepare test table and oda data source and data set design. In most case,
	 * only one data source needs to be defined.
	 * 
	 * @throws Exception
	 */
	private void prepareDataSource( ) throws Exception
	{
		DataSourceInfo dataSourceInfo = getDataSourceInfo( );
		if ( dataSourceInfo != null )
		{