Refactor `channel_update` processing logic into local fns

Refactor `channel_update` processing logic into local fns

In the next commit we'll move to checking `channel_update`s in
three steps - first we check if the `channel_update` is new and the
latest for a channel, then we check the signature, and finally we
update our local state. This allows us to avoid holding a lock on
`NetworkGraph::channels` while validating the message signature.

Here we do a quick prefactor to make that simpler - moving the
validation logic of `channel_update` that we'll do in step one (and
repeat in step three) into a local function. We also take this
opportunity to do one static check unlocked which we had been doing
while holding a `channel` lock.