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/DataEngine.java
* prepare.
	 * 
	 * @deprecated
	 * @see org.eclipse.birt.data.engine.api.DataEngine.prepare(
	 *      IQueryDefinition querySpec, Map appContext)
	 */
	public static String DATASET_CACHE_OPTION = "org.eclipse.birt.data.engine.dataset.cache.option";
	
	/**
	 * Indicates whether data set cache function need to be used:
	 * 
	 * Negative integer: Enable data set cache. The row limit will not be set and all data set rows are fetched.
	 * 
	 * Positive integer: Enable data set cache. This row limit will be used for ALL
     * data sets, regardless of settings on individual data set designs.
     * 
     * Zero:Disable data set cache regardless of report design settings and
	 * DataEngineContext settings. 
	 */
	public static String DATA_SET_CACHE_ROW_LIMIT = "org.eclipse.birt.data.cache.RowLimit";
	public static String DATA_SET_CACHE_DELTA_FILE = "org.eclipse.birt.data.cache.DeltaFile";
	
	
	/**
	 * Creates a new instance of DataEngine, using the specified
	 * DataEngineContext as its running environment
	 * 
	 * @param context,
	 *            When this value is null, a default context will be used. The
	 *            default context is DataEngineContext.MODE_DIRECTPRESENT.