Persist fork_choice on stop and load on restart

[twoeths]

Problem description

Right now when we restart a lodestar beacon node, we completely ignore the old forkchoice, and we start from the head of the finalized state. This cause the node to be out of date compared to EL, and other CL peers

Solution description

See lighthouse reference implementation:

• load fork_choice: https://github.com/sigp/lighthouse/blob/e5b4983d6baf85770fe4539a565d8a2dd462bc53/beacon_node/beacon_chain/src/beacon_chain.rs#L608
• persist fork_choice: https://github.com/sigp/lighthouse/blob/af9cae4d3e32fadbc022c494a8cad7bbc6012398/beacon_node/beacon_chain/src/beacon_chain.rs#L7503

Additional context

No response