Function jormungandr::blockchain::compare_against

pub fn compare_against(
    storage: &Storage,
    current: &Ref,
    candidate: &Ref
) -> ComparisonResult

chose which of the two Ref is the most interesting to keep as a branch

i.e. if the two Ref points to the same block date: this allows to make a choice as to which Ref ought to be our preferred choice for a tip.