[Maria-developers] subquery cache for review

Hi! I have problem with automatic commits sending so sends the diff here (sorry, will fix it tomorrow). I also have thought about renaming sql/sql_expression_cache.* to sql/item_expression_cache and moving the item also there but I am not sure if it is better. Also I am not sure that Item_cache_wrapper is the best name but Item_expression_cache_wrapper IMHO is too long. I re-made 5.3-mwl-66 so it need re-branching (not pulling) if you need to look at it.