Fork choice consensus updates

[dapplion]

This issue is a remainder that the Fork choice spec has and will be patched against some attacks. Implementation is not immediate but must be completed prior to the merge.

• always atomically update justified and finalized ethereum/consensus-specs#2727
• Proposer LMD Score Boosting ethereum/consensus-specs#2730

This changes should be optional behind a flag. See sigp/lighthouse#2838 for a great overview of the required changes

[dapplion]

Āina-kāri release https://github.com/ethereum/consensus-specs/releases/tag/v1.1.6 is out. We should implement this changes by mid January 2022.

From ethereum/consensus-specs#2733
Q: CL clients are expected to implement #2730 at any time between now and the merge? Is there any time target or just whenever possible?
A: I'll put some notes in the release, but ideally, these changes are on merge testnets by roughly end of January and released on the normal release cycle at or before the Merge itself