channeldb: add new FetchOtherNode method to ChannelEdgeInfo
channeldb: add new FetchOtherNode method to ChannelEdgeInfo

In this commit, we add a new method to the ChannelEdgeInfo that will
allow the path finding logic to get the node opposite the pivot node
without first creating a new db transaction. The new method is able to
use an existing db transaction, or create a new one if needed.

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