bug: lastBatchHash only stored in-memory leading to mismatch after restart

[gupadhyaya]

the lastBatchHash is stored only in-memory: https://github.com/rollkit/rollkit/blob/main/block/manager.go#L446
this leads to following error if the node stops and restarts

6:24PM ERR error while retrieving batch error="rpc error: code = Unknown desc = batch hash mismatch: lastBatchHash = bb38c42f9c5965ecc3efffd764773e0cc2f8abb850ba1b279cb7579c6ab55b6e, req.LastBatchHash = " module=BlockManager

solution is to persist the lastBatchHash to store and load it on restart.