multi: move failed attempt cfg option to the router subsytem

Previously we had db and application logic mixed on the db level.
We now move the config option KeepFailedPaymentAttempts to the
ChannelRouter level and move it out of the db level.

routing/router.go

@@ -295,6 +295,10 @@ type Config struct {
 	// TrafficShaper is an optional traffic shaper that can be used to
 	// control the outgoing channel of a payment.
 	TrafficShaper fn.Option[htlcswitch.AuxTrafficShaper]
+
+	// KeepFailedPaymentAttempts indicates whether to keep failed payment
+	// attempts in the database.
+	KeepFailedPaymentAttempts bool
 }
 
 // EdgeLocator is a struct used to identify a specific edge.