Continue attesting when web3 goes down

[arnetheduck]

When the connection to the web3 server is lost, it remains possible and valid to keep voting for the last known head for some time (1 epoch? up to 2?) - using an old head/target/source still nets a rewards and remains a valid vote.

Ditto when syncing: after optimistic sync, it's sufficient that the execution layer confirms any block within the last 1-2 epochs to start attesting to that block.