routing: don't re-validate a channel update's edge if it already exists
routing: don't re-validate a channel update's edge if it already exists

By avoiding re-validating the channel edge, we avoid wasted network
bandwidth and queries.

LightningNetwork/lnd,ddustin/lnd,LightningNetwork/lnd,aakselrod/lnd,ddustin/lnd,aakselrod/lnd