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
/**
	 * Remember before requesting any service, this function must be called in
	 * advance to make sure using current data source and data set.
	 * 
	 * @param dataSourceDesign
	 * @param datasetDesign
	 */
	public void setDataSourceAndDataSet(
			IBaseDataSourceDesign dataSourceDesign,
			IBaseDataSetDesign dataSetDesign, Collection parameterHints, Map appContext )
	{
		this.dataSourceDesign = dataSourceDesign;
		this.dataSetDesign = dataSetDesign;
		this.parameterHints = parameterHints;
		this.appContext = appContext;
		if ( needToMerge )
		{
			try
			{
				this.mergeDelta( );
			}
			catch ( BirtException e )
			{
			}
		}
	}
	/**
	 * @return
	 */
	public boolean doesSaveToCache( )