forkchoice: only store descendant of finalized node

[twoeths]

          @tuyennhv please research the following:

• when changing finalized create a new nodes vector that includes only descendant of finalized
• keep a mapping of the node index changes and update all indexes

With this we pay a higher ocassional cost, but then we never have to check if descedant of finalized. My concern is that finalized checkpoint can change at times where the node is hot. But it should be worth it to benchmark how expensive this remapping operation is

Originally posted by @dapplion in https://github.com/ChainSafe/lodestar/issues/5852#issuecomment-1676299054

[wemeetagain]

@twoeths is this still relevant?

[twoeths]

feature was implemented