multi: add debug logs for edge policy flow

This commit adds more logs around the ChannelUpdate->edge policy process
flow.

graph/db/models/channel_edge_policy.go

@@ -1,6 +1,7 @@
 package models
 
 import (
+	"fmt"
 	"time"
 
 	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
@@ -113,3 +114,10 @@ func (c *ChannelEdgePolicy) ComputeFee(
 
 	return c.FeeBaseMSat + (amt*c.FeeProportionalMillionths)/feeRateParts
 }
+
+// String returns a human-readable version of the channel edge policy.
+func (c *ChannelEdgePolicy) String() string {
+	return fmt.Sprintf("ChannelID=%v, MessageFlags=%v, ChannelFlags=%v, "+
+		"LastUpdate=%v", c.ChannelID, c.MessageFlags, c.ChannelFlags,
+		c.LastUpdate)
+}