Skip `channel_update` signature checks if we have a newer state

Skip `channel_update` signature checks if we have a newer state

`channel_update` messages already have their signatures checked
with the network graph write lock held, so there's no reason to
check the signatures before doing other quicker checks first,
including checking if we're already aware of a newer update for the
channel.

This reduces common-case CPU usage as `channel_update`s are sent
rather liberally over the p2p network to gossip them.