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
* 				needed from DtE to produce a set of query results.
	 * @return		The <code>IPreparedQuery</code> object that contains a prepared 
	 * 				query ready for execution.
	 * @throws 		DataException if error occurs during the preparation of querySpec
	 */
	abstract public IPreparedQuery prepare( IQueryDefinition querySpec )
			throws BirtException;
	/**
	 * Verifies the elements of a report query spec,
	 * and provides a hint and application context object(s) to the 
	 * query to prepare and optimize an execution plan.
	 * <br>This has the same behavior as the 
	 * prepare( IQueryDefinition querySpec ) method,
	 * with an additional argument for an application to pass in 
	 * a context map to the underlying data provider, 
	 * e.g. an ODA run-time driver.
	 * @param	querySpec	Specifies
	 * 				the data access and data transforms services
	 * 				needed from DtE to produce a set of query results.
	 * @param appContext	The application context map for 
	 * 				preparation and execution of the querySpec; 
	 * 				could be null.
	 * @return		The <code>IPreparedQuery</code> object that contains a prepared 
	 * 				query ready for execution.
	 * @throws 		BirtException if error occurs during the preparation of querySpec
	 * @since		2.0
	 */
	abstract public IPreparedQuery prepare( IQueryDefinition querySpec, 
	        								Map appContext )
			throws BirtException;