Bug 180189 Support in memory data set cache.
Currently, Design-Time Data Cache is stored on disks. By performance tests using ramdisk, we found that caching in physical memory can improve the performance of RunAndRender by 30%.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/executor/DataSetCacheManager.java
* @param appContext2
	 * @return
	 */
	private String getCacheConfig( Map context )
	{
		return context.get( DataEngine.DATA_SET_CACHE_DELTA_FILE ) == null
				? null
				: String.valueOf( context.get( DataEngine.DATA_SET_CACHE_DELTA_FILE ) );
	}
	/**
	 * 
	 * @param dataFile
	 * @param list
	 * @param metaFile
	 * @throws DataException
	 * @throws IOException
	 * @throws BirtException
	 * @throws ClassNotFoundException
	 */
	private void mergeDeltaToFile( File dataFile, ArrayList list, File metaFile )
			throws DataException, IOException, BirtException,
			ClassNotFoundException
	{
		MergeUtil merge = new MergeUtil( dataFile, metaFile );
		
		IResultIterator iterator = getResultIterator( list );
		if ( iterator != null )
		{
			ResultObject ro;
			while ( iterator.next( ) )