[WIP] Optimise no-op builder

[michaelsproul]

This PR updates the empty-payload builder so that it can build payloads without receiving a prior validator registration.

For blockdreamer I need a builder that will return a dummy payload at every slot as quickly as possible. Bypassing the registration step is essential, as blockdreamer never has the ability to sign as the true proposer.

I've added a flag --default-fee-recipient which specifies a fee recipient to be used in when the registration is missing.

[michaelsproul]

The issue I'm having currently is that the call to get_params is taking way too long, around 2 seconds on Goerli. Most of this seems to be for loading the head state to get the prev_randao here:

https://github.com/realbigsean/lighthouse/blob/d35934df2c7cdcb692e6f1c09d9f387f5d9e8972/beacon_node/execution_layer/src/test_utils/mock_builder.rs#L359-L364

It seems like a huge pain to have to load a state to get the prev_randao, and I think we're probably not the only ones to notice this. I think it would be great to have a standard API on the beacon node like /eth/v1/beacon/states/{state_id}/randao/{epoch} to accelerate this. Then the process for dummy payload creation could be:

• Fetch head beacon block: this determines block_number and parent_hash (if we want to check that). It gives us the state_root of the head.
• Using state_root: fetch the randao mix for the current epoch. The state_root should match the head so this call should be fast.

Other/alternative avenues for optimisation:

• Try SSZ requests in get_params rather than JSON (alternative)
• Avoid transporting the exec payload by fetching a blinded block for the head. There's no endpoint for this, but I think it would be a good candidate for standardisation and early impl in Lighthouse.
• Cache these parameters ahead of time (every slot?)
• Cache these parameters between requests. For my purposes I'm likely to request the same dummy payload several times, once for each connected BN.

[michaelsproul]

New endpoints proposed here:

• Endpoint for fetching blinded blocks ethereum/beacon-APIs#241
• Endpoint for state RANDAO mix ethereum/beacon-APIs#242

[michaelsproul]

This is working well for my purposes, fetching data from the BN only takes ~5ms now.

It's a pretty big pile of hacks though, so I'm happy for this to remain as an open PR.

The version of Lighthouse I'm depending on has all of these changes merged into it:

• your mock-builder-getter branch: https://github.com/realbigsean/lighthouse/tree/mock-builder-getter
• my PR for the new APIs: [Merged by Bors] - Blinded block and RANDAO APIs sigp/lighthouse#3571
• my PR for skip_randao_verification: [Merged by Bors] - Implement skip_randao_verification and blinded block rewards API sigp/lighthouse#3540

[realbigsean]

It's a pretty big pile of hacks though, so I'm happy for this to remain as an open PR.

I think it's great! The hacks actually fit right in because mock-relay itself is a hack 😂