Bug 177870 Allow design-time data set cache to be controlled solely by AppContext
This is to request that BIRT allows data set caches to be controllable solely by runtime parameters/settings, without the application having to update the report design to set data set cache preferences. Since BIRT DataEngine already reads request Application Context to determine if data set cache should be disabled, we will expand AppContext to allow the application to enable data set cache and to set max row count. The following new application context key will be introduced: org.eclipse.birt.data.cache.RowLimit The value of this app context is expected to be an integer. Its value can be: 0: Disable data set cache regardless of report design settings and DataEngineContext settings. Positive integer: Enable data set cache. This row limit will be used for ALL data sets, regardless of settings on individual data set designs. If this key is absent in the appcontext, or if its value is not a non-negative integer (including NULL), we will following current rules to determine cache settings. As part of this change, we will also deprecate existing app context key "org.eclipse.birt.data.engine.dataset.cache.option". Its declaration in DataEngine class should be marked deprecated. We will continue to read and handle its value, but the user is encouraged to use the new key "org.eclipse.birt.data.cache.RowLimit" (by setting its value to 0).
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