fix: Comments about SyncBatch vs L1 and wait for batch persisted (BFT-474)

[aakoshh]

What ❔

• Fixes the comments I made about SyncBatch::proof requiring L1 inclusion - that was based on a misunderstanding that the proof was a Merkle proof starting from the L1 state root hash, rather than a field inside the zksync system contract on L1. We only need an L1 client to validate the proof, but we can produce it offline as soon as we have the commitment locally.
• Changed the loop in AttesterRunner so that it waits for persisted state of the BatchStore to indicate that the next batch number is available. This will be when the SyncBatch can be constructed, so it's when the commitment is ready too (or will be, not currently) and we're ready to sign the payload.
• Removed BatchStore::last_batch_number; I wanted to use persisted.last, which can be None in the beginning until SyncBatch::proof is available, with a fallback to PersistentBatchStore::last_batch which just looks at the latest l1_batches record, but I wasn't sure whether a next batch can be created before the commitment for the previous one is available, which would cause a regression in the value returned. Instead we just use BatchStore::wait_for_persisted(0) to tell us what the starting value is after the first batch is ready.

Why ❔

To remove misleading comments, and to reduce unneeded polling of the database itself - now we wait until the memory indicates it's worth hitting the DB.