RareNFT and nonce issue

[mtomassoli]

The POC for the RareNFT assumes that the nonce doesn't change between the off-chain reading and the on-chain attack but that's unrealistic when there are other users.
Luckily, the nonce and the latest token id are in sync, and the latest id can be deduced on-chain in O(logN) time, where N is the number of users who called mint after we read nonce.
edit: Actually, MockERC721.nftId is public so we can just read it in O(1).