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/executor/cache/CacheUtil.java
* @param folder
	 * @return String that contains the last time doing caching or merging
	 * @throws DataException
	 * @throws ClassNotFoundException
	 */
	public static String getLastTime( String folder ) throws IOException,
			DataException, ClassNotFoundException
	{
		try
		{
			File file = new File( folder + "\\time.data" );
			if ( !file.exists( ) )
			{
				return null;
			}
			FileInputStream fis = new FileInputStream( file );
			ObjectInputStream ois = new ObjectInputStream( fis );
			String lastTime = (String) ois.readObject( );
			fis.close( );
			ois.close( );
			return lastTime;
		}
		catch ( IOException e )
		{
			throw new DataException( ResourceConstants.DATASETCACHE_SAVE_ERROR,
					e );
		}
	}
	/**
	 * To save the current time doing caching or merging