[Sequencer] Starting L1 message cursor

[greged93]

Currently on initial work of the sequencer, we do not set the L1 message cursor, meaning it will by default start at 0. We should have a way for the sequencer to start at the latest not executed L1 message.

[Thegaram]

Just to emphasize: It must be ensured that we find the highest queue index that has been processed in the ancestor chain, and start packing messages from the next queue index (no skipping).

In l2geth, after processing any L2 block (build or validate), we store the highest queue index processed up to and including this block. This way, for the next block, we can easily retrieve this value using the parent hash.

This information might not be obtainable just by looking at the parent block; it might be possible that we last processed L1 messages 10 blocks ago, for example.