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/QueryExecutor.java
{
	}
	/**
	 * Executes the ODI query to reproduce a ODI result set
	 * @param eventHandler 
	 * 
	 * @return
	 */
	abstract protected IResultIterator executeOdiQuery(
			IEventHandler eventHandler ) throws DataException;
	/**
	 * @param context
	 */
	void setAppContext( Map context )
	{
		queryAppContext = context;
	}
	/**
	 * Prepare Executor so that it is ready to execute the query
	 * 
	 * @param outerRts
	 * @param targetScope
	 * @throws DataException
	 */
	void prepareExecution( IQueryResults outerRts, Scriptable targetScope )
			throws DataException
	{
		if ( isPrepared )
			return;
		this.parentScope = targetScope;