FCU ignored after payload is processed
When a payload is sent to eth1, it processes and validates fine
but then `fcu` is called, and says it is `updating to the old head` and thus ignoring the request

Shouldn't behave like that, but it is because of the payload processing
In the `new_payload`, we are also persisting the header to the db 
https://github.com/status-im/nimbus-eth1/blob/ea74e035ab019619fc00b46ab97496b502a188dd/nimbus/core/chain/forked_chain.nim#L88-L95

Then when the fcu call takes place, it checks if the header is there in the db, and it is thus ignoring the fcu call 
https://github.com/status-im/nimbus-eth1/blob/ea74e035ab019619fc00b46ab97496b502a188dd/nimbus/beacon/api_handler/api_forkchoice.nim#L158-L163

A similar issue arising from the same persistence of header #2588 