Restrict the number of preconfers in the lookahead

[dantaik]

In theory, if all L1 validators opt into preconfirmation, the number of empty slots (slots without preconfers) in the lookahead would drop to zero. As a result, each preconfer would be limited to proposing Taiko blocks over a small number of consecutive slots. This would mean that each proposer must submit smaller L2 blocks when there aren't enough transactions in the L2 mempool, making the protocol less cost-effective due to underutilized blob space.

One idea to address this is to only allow a validator's preconfer to be selected only if slot_id % N == 0 in the lookahead. Currently, N is set to 1, meaning every validator's preconfer is enabled.

However, some may argue that if a large number of validators have opted in, the transaction count per L2 block would already be substantial, making this feature less necessary. Nonetheless, I wanted to bring this up in the hope that others might consider it and potentially propose even better optimizations for the lookahead implementation.