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/api/DataEngine.java
* @return an instance of DataEngine under specified context
	 */
    public static DataEngine newDataEngine( DataEngineContext context )
    {
    	try
		{
			return newDataEngine( null, context );
		}
		catch ( BirtException e )
		{
			
			throw new RuntimeException( e );
		}
    }
    
    /**
     * @param context
     * @return
     * @throws BirtException 
     */
    public static DataEngine newDataEngine( PlatformConfig platformConfig,
			DataEngineContext dataContext ) throws BirtException
	{
		Platform.startup( platformConfig );
		Object factory = Platform.createFactoryObject( IDataEngineFactory.EXTENSION_DATA_ENGINE_FACTORY );
		if ( factory instanceof IDataEngineFactory )
		{
			return ( (IDataEngineFactory) factory ).createDataEngine( dataContext );
		}
		else