Set the new `dont_forward` bit in channel_update messages

[TheBlueMatt]

See lightning/bolts#999 should be super trivial to set.

[jirijakes]

I am looking at this as my potential first contribution and would like to clarify when the dont_forward flag should be set to 1. To my understanding, the flag should be set to 1 only in get_channel_update_for_unicast when short_channel_id does not exist but latest_inbound_scid_alias exists. In all other cases, it should be 0. Is that correct?

rust-lightning/lightning/src/ln/channelmanager.rs

Lines 2333 to 2346 in 41a6c67

	/// Gets the current channel_update for the given channel. This does not check if the channel
	/// is public (only returning an Err if the channel does not yet have an assigned short_id),
	/// and thus MUST NOT be called unless the recipient of the resulting message has already
	/// provided evidence that they know about the existence of the channel.
	/// May be called with peer_state already locked!
	fn get_channel_update_for_unicast(&self, chan: &Channel<<SP::Target as SignerProvider>::Signer>) -> Result<msgs::ChannelUpdate, LightningError> {
	log_trace!(self.logger, "Attempting to generate channel update for channel {}", log_bytes!(chan.channel_id()));
	let short_channel_id = match chan.get_short_channel_id().or(chan.latest_inbound_scid_alias()) {
	None => return Err(LightningError{err: "Channel not yet established".to_owned(), action: msgs::ErrorAction::IgnoreError}),
	Some(id) => id,
	};
	self.get_channel_update_for_onion(short_channel_id, chan)
	}

[TheBlueMatt]

No, its unrelated to the SCID, but rather should be set based on the channel is_public

[TheBlueMatt]

Once we do this, we should print a big fat WARNING log in ChannelManager if we receive a channel_update with the dont_forward bit set but for a channel we don't have - telling the user they may want to look at whether they are passing the correct tx index to chain::Confirm::transactions_confirmed.