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.
Because we expose the internals we don't need a method to log
their contents anymore, and get_addresses can now avoid copying as
we expose the RwLock directly
