L2: Use state transition to calculate state diff len

[tomip01]

If #2825 is merged, we will have some edge cases where we are calculating the state diff size to be bigger than it will be. This is linked to this comment #2825 (review) .

The explanation is that when calculating the account state diff, we compare it to the state after executing the last transaction in the block, rather than the initial state of the block. This can lead to incorrect differences being counted when a value is modified from a to b and then back to a in two separate transactions.