Fix a few new (and one old) issues in the new channel_update

We need to always set lowest_inbound_channel_fees to None if there
are no channels (and we should ignore the channels which are
disabled for the purpose of lowest-fee calculations). Further, we
cannot unwrap the channel lookups as they may be for channels which
are not related to the channel we are processing an update for.
Finally, we can satisfy borrowck much easier since things are on
self instead of on a MutexGuard.