Insert unsafe blocks in order while performing EL sync

[trianglesphere]

Geth is not able to cleanly handle gaps in blocks while performing EL sync. During snap sync it logs err chain gapped, during full sync, it will log chain reorg & return early out of newPayload / FCU.

The way to implement this is to use the unsafe payload queue and the request/response protocol to backfill missing unsafe payloads. We should only be doing this close to tip so the current request/response protocol will work. Just the native unsafe payload queue will help in the case the unsafe blocks are received out of order.

In the case that op-node is restarted while op-geth stays up, op-node may want to start inserting unsafe heads right after op-geth's current unsafe head (though IDK if there is a way to get that, maybe we have to use eth.syncing instead.)