Dont apply NetworkUpdate::ChannelUpdate in an observable way

[TheBlueMatt]

If we apply ChannelUpdate to our graph in an observable way, the node that generated that update can detect that we were the sender of the payment, making a rather substantial privacy leak. We should instead keep any updates local to the given payment.

[tnull]

Mh, after having a closer look at this it seems we can just avoid applying the updates and be done with it, as we avoid reusing any PaymentParameters::previously_failed_channels anyways?

That is, it doesn't make a lot of sense handing the channel updates through, verifying them, and accounting for them in routing if we'd never end up using the failed, updated channels anyways?

[TheBlueMatt]

Yea, makes sense to me.