Rollups: Fix MapArray overwriting bug

Make MapArray store the max index ever used, rather than the current size. We use MapArray to store commitments, and we don't want to re-use a commitment index after the commitment has been rejected. If we do this, then it would be unclear which commitment a future rejection (or commitment) was referring to.

I considered removing MapArray entirely, and just using the hash of the commitment as the map key, but that was actually a lot of work, because we wouldn't be able to find a commitment just by looking at the block.