Do not attempt to handle unicast channel_update in NetworkGraph

[devrandom]

We should not attempt to hand the update to NetworkGraph if the !chan.config.announced_channel, so we don't see spurious "[lightning::ln::peer_handler:804] Got Err handling message, ignoring because Couldn't find channel for update" errors in the logs.

[TheBlueMatt]

I'm not sure what we can practically do about this - the NetGraphMsgHandler doesn't have any ideas what channels are ours and what aren't. We can tweak the error message to be "Couldn't find channel for update or channel is private".

[devrandom]

Since we hand it to the ChannelManager first, it can tell us whether the channel is private, so we can skip handing it to the network graph.

[TheBlueMatt]

We can now do this because there's a dont-forward bit in the update.