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/executor/cache/ResultObjectUtil.java
* proper read/write method will be called. This method must be called at
	 * first when any actual read/write action is taken. Since multi thread
	 * might call DtE at the same time, an instance needs to be new to
	 * correspond to different metadata.
	 * 
	 * @param rsMetaData
	 * @throws DataException
	 */
	public static ResultObjectUtil newInstance( IResultClass rsMetaData )
	{
		ResultObjectUtil instance = new ResultObjectUtil( );
		int length = rsMetaData.getFieldCount( );
		instance.typeArray = new Class[length];
		for ( int i = 0; i < length; i++ )
		{
			try
			{
				instance.typeArray[i] = rsMetaData.getFieldValueClass( i + 1 );
			}
			catch ( DataException e )
			{
				
			}
		}
		instance.columnCount = rsMetaData.getFieldCount( );
		instance.rsMetaData = rsMetaData;
		
		return instance;
	}
	/**