MEV protection

Consider having the start block be block.number + 1. This prevents "MEV"-style attacks which would subscribe and unsubscribe in the same transaction for "free" (less gas costs) to pretend they were subscribers for a short time.

https://github.com/edgeandnode/subscription-payments-contract/blob/7b0368e4da25ece37440e0999fbeee652ad8b103/contracts/Subscriptions.sol#L85