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

I don’t think copying the entire multi_index table is a good practice because of several reason:
— you can’t copy _items_vector, because there’s a std::unique_ptr inside each of its element owning the corresponding cache
— changing from std::unique_ptr to std::shared_ptr to allow copying is inefficient, which will only result in more memory and time overhead.

workaround: put your multi_index instance as member of your class, so that your function «`get_multi_index_of «` should only return the pointer of your multi_index instance.