Check SnapshotMap with multiple updated

[ethanfrey]

Double check how SnapshotMap handles multiple updates of one entry in one block.

If we move from 2 => 3 and then 3 => 5 in one block, we should store a diff of {old: 2, new: 5}.
That means queries at h should return 2 and queries at h+1 should return 5.

I don't think this is fully the case (I think it keeps {old: 2, new: 3} which is almost but not exactly correct

[ethanfrey]

Update: https://github.com/CosmWasm/cosmwasm-plus/blob/ee98e0167123a35bbad21f7309138252a959676d/packages/storage-plus/src/snapshot.rs#L129-L141

This is a larger error, we overwrite, so the case above would store {old: 3} and thus multiple updates in a block could trick the snapshotter.