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
* @param expr
	 * @return
	 */
	private String populateGroupName( int i, String expr )
	{
		String groupName;
		if ( expr.trim( ).equalsIgnoreCase( "row[0]" )
				|| expr.trim( ).equalsIgnoreCase( "row._rowPosition" )
				|| expr.trim( ).equalsIgnoreCase( "dataSetRow[0]" )
				|| expr.trim( )
						.equalsIgnoreCase( "dataSetRow._rowPosition" ) )
		{
			groupName = "_{$TEMP_GROUP_" + i + "ROWID$}_";
		}
		else
		{
			groupName = "_{$TEMP_GROUP_" + i + "$}_";
		}
		return groupName;
	}
	/**
	 * Get the computed column instance according to the group type.If group has
	 * interval, return GroupComputedColumn, otherwise return normal computed
	 * column.
	 * 
	 * @param cx
	 * @param groupSpecs
	 * @param i
	 * @param src
	 * @param expr