replace cursor.equals with cursor.compare, rework session.truncate, ref WT-314

Michael, the one thing I think you might want to consider here: cursor.equals used to work (maybe?) for other cursor types than file and table (if you recall, cursor.equals retrieved raw cursors and then did a byte comparison, that said, I don't think we ever actually tested it ). I don't see much use for cursor comparison in other types of objects, so I didn't try and make cursor.compare work for anything other than file or table types. Let me know if you disagree with that.