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/impl/DataEngineSession.java
{
	private Map context;
	private Scriptable scope;
	private DataSetCacheManager dataSetCacheManager;
	private CacheUtil cacheUtil;
	
	/**
	 * Constructor.
	 * @param engine
	 */
	public DataEngineSession( DataEngineContext context, DataEngine engine )
	{
		this.context = new HashMap();
		
		this.dataSetCacheManager = new DataSetCacheManager( context.getTmpdir( ), engine );
		this.cacheUtil = new CacheUtil( context.getTmpdir( ) );
		
		this.scope = context.getJavaScriptScope( );
		
		Context cx = Context.enter( );
		if ( this.scope == null )