[EOSIO/eos] Bug: elements of _items_vector are not updated when multi_index is moved to new instance (#5317)

I agree, but in this case are only cached items which would not be available to the new instance. The state of `_items_vector` in both instances (old and new) remains valid so nothing breaks. The only drawback is that the instance index moved to has to (lazily) load again already cached items. Its temporarily solution but it would fix current broken interface.