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
* time the data will be retrieved again from data source to update cache.
	 * 
	 * @param dataSource, which is associated with the data set
	 * @param dataSet, which cache needs to be cleared
	 * @throws BirtException
	 */
	abstract public void clearCache( IBaseDataSourceDesign dataSource,
			IBaseDataSetDesign dataSet ) throws BirtException;
	
	/**
	 * Verifies the elements of a report query spec
	 * and provides a hint to the query to prepare and optimize 
	 * an execution plan.
	 * The given querySpec could be a <code>IQueryDefinition</code> 
	 * (raw data transform) spec  
	 * based on static definition found in a report design.
	 * <p> 
	 * This report query spec could be further refined 
	 * during engine execution after having resolved any related
	 * runtime condition.
	 * For example, a nested report item might not be rendered based
	 * on a runtime condition.  Thus its associated data expression
	 * could be removed from the report query defn given to 
	 * DtE to prepare.
	 * <p>
	 * During prepare, the DTE does not open a data set. 
	 * In other words, any <code>beforeOpen</code> script on a data set will not be
	 * evaluated at this stage. 
	 * @param	querySpec	Specifies
	 * 				the data access and data transforms services