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
* @return an instanceof IQueryResults
	 * @throws BirtException
	 */
	public abstract IQueryResults getQueryResults( String queryResultID )
			throws BirtException;
    
	/**
	 * Provides the definition of a data source to Data Engine. A data source
	 * must be defined using this method prior to preparing any report query
	 * that uses such data source. <br>
	 * Data sources are uniquely identified name. If specified data source has
	 * already been defined, its definition will be updated with the content of
	 * the provided definition object.
	 */
	abstract public void defineDataSource( IBaseDataSourceDesign dataSource ) 
			throws BirtException;
	/**
	 * Provides the definition of a data set to Data Engine. A data set must be
	 * defined using this method prior to preparing any report query that uses such data set.
	 * <br>
	 * Data sets are uniquely identified name. If specified data set has already
	 * been defined, its definition will be updated with the content of the provided definition object.
	 */
	abstract public void defineDataSet( IBaseDataSetDesign dataSet ) 
			throws BirtException;
	
	/**
	 * If user wants to use the latest data from data set rather than local
	 * cache, user needs to call this clearCache method to delete the cache
	 * content from update local environment. After it is called, in the next