routing: modify path finding routines use new EdgeInfo/EdgePolicy

This commit modifies the path finding routines to properly use the new
channel edge related API exposed by the database. Additionally, a new
type `ChannelHop` has been introduced which couples an edges routing
policy with the capacity and origin chain of the channel.

channeldb/graph.go

@@ -1075,7 +1075,7 @@ type ChannelAuthProof struct {
 	BitcoinSig1 *btcec.Signature
 
 	// BitcoinSig2 is the signature using the public key of the second node
-	// that was used in the channel's mult-sig output.
+	// that was used in the channel's multi-sig output.
 	BitcoinSig2 *btcec.Signature
 }