Async block creation post-merge

[MariusVanDerWijden]

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async block creation only creates one block rn and does not update the pending block when new transactions are rolling in

[allboxes]

This would be great to see soon after the merge.
CLs that send early FCUs will have potentially empty or nearly empty blocks if they propose after a builder block or after CL that does a very late FCU.

This might push more validators to centralize on builders or CLs with late FCU.

[MariusVanDerWijden]

fixed long time ago