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
* When mode is MODE_PRESENTATION, the reader stream of archive will be used.
	 * When mode is DIRECT_PRESENTATION, the archive will not be used.
	 * When mode is PRESENTATION_AND_GENERATION, both the write stream and the read 
	 * steram of archive will be used. 
	 * 
	 * @param mode
	 * @param scope
	 * @param reader
	 * @param writer
	 * @return an instance of DataEngineContext
	 */
	public static DataEngineContext newInstance( int mode, Scriptable scope,
			IDocArchiveReader reader, IDocArchiveWriter writer )
			throws BirtException
	{
		return new DataEngineContext( mode, scope, reader, writer );
	}
	
	/**
	 * @param mode
	 * @param scope
	 * @param reader
	 * @param writer
	 * @throws BirtException
	 */
	private DataEngineContext( int mode, Scriptable scope,
			IDocArchiveReader reader, IDocArchiveWriter writer )
			throws BirtException
	{
		if ( !( mode == MODE_GENERATION