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
* @param createSql
	 * @param dataFileName
	 * @throws Exception
	 */
	private void prepareTestTable( String tableName, String createSql,
			String dataFileName ) throws Exception
	{		
		if ( dataSourceInstance == null )
			dataSourceInstance = JDBCDataSource.newInstance( );
		
		
		this.dataSourceInstance.createTable( tableName,
				createSql,
				true );
		
		
		this.dataSourceInstance.populateTable( tableName,
				getInputFolder(dataFileName) );
	}
	
	/**
	 * Normally, an API test case will be based on a particular data source, so
	 * before test begins, test case provides which data source will be used and
	 * then its data will be prepared and meantime its data source and data set
	 * will be defined in DataEngine. If the return value is null, there is no
	 * any datasource and dataset is defined in dataEngine, which means other
	 * methods such as newDataSet and newReportQuery can not be used.
	 * 
	 * @return which data source will be used in this test case
	 */