Bug 180842 Config memory buffer size in data engine execution.
To tuning the performance, it is necessary for DtE to expose API for clients to config the size of memory it used to store the final result set. A new AppContext string, "org.eclipse.birt.data.cache.memory.size" is proposed. The new appcontext entry only accept positive integer as input, the unit of which would be MB.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/impl/IQueryExecutor.java
int getNestedLevel( );
	/**
	 * @return
	 */
	IDataSourceInstanceHandle getDataSourceInstanceHandle( );
	/**
	 * @return the data set associated with this executor
	 */
	DataSetRuntime getDataSet( );
	/**
	 * @param nestedCount
	 * @return nested data set
	 */
	DataSetRuntime[] getNestedDataSets( int nestedCount );
	/**
	 * @return result set of underlying ODI layer
	 */
	IResultIterator getOdiResultSet( );
	/**
	 * @return meta data of data set
	 * @throws DataException
	 */
	IResultMetaData getResultMetaData( ) throws DataException;
	/**
	 * close
	 */
	void close( );
	
}