split out from ZBXNEXT-908 (5)

open a graph, save it (without changing anything).
we do update query with all fields in the graphs table. even worse, we delete all graph items, then create them one by one, doing select from ids/update ids for every one of them.
+ if possible, we shouldn't do update in this case;
+ we shouldn't delete all items & recreate them;
+ when creating items (for a new graph, for example), maybe it's possible to increase ids only once and insert all graph items in one go ?