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/api/DataEngineContext.java
* @param streamType
	 * @return relative path, notice in reading data from file, directory can
	 *         not be created.
	 */
	private static String getPath( String streamID, String subStreamID, int streamType )
	{
		if ( streamType == VERSION_INFO_STREAM )
			return "/DataEngine/VesionInfo";
		
		String relativePath = null;
		switch ( streamType )
		{
			case DATASET_DATA_STREAM :
				relativePath = "DataSetData";
				break;
			case DATASET_META_STREAM :
				relativePath = "ResultClass";
				break;
			case EXPR_VALUE_STREAM :
				relativePath = "ExprValue";
				break;
			case EXPR_ROWLEN_STREAM :
				relativePath = "ExprRowLen";
				break;
			case EXPR_META_STREAM :
				relativePath = "ExprMetaInfo";
				break;
			case GROUP_INFO_STREAM :
				relativePath = "GroupInfo";
				break;