This ticket assumes that there are no forks.

Depends on: #244

Changes:

• When receiving a new payload, we should not set the new head or update the canonical chain, but instead just validate the payload and update the state trie (but the canonical state_root will not change since we don't update the head).
• When receiving a forkchoiceUpdated call, we should update the head (latest block number) and the canonical chain if it is valid (and the rest of the fields, like safe block).
• We should be able to store a block that is not canonical