channeldb+routing: add new interface method TerminalInfo

This commit adds a new interface method `TerminalInfo` and changes its
implementation to return an `*HTLCAttempt` so it includes the route for
a successful payment. Method `GetFailureReason` is now removed as its
returned value can be found in the above method.

routing/control_tower.go

@@ -31,13 +31,14 @@ type dbMPPayment interface {
 	// InFlightHTLCs returns all HTLCs that are in flight.
 	InFlightHTLCs() []channeldb.HTLCAttempt
 
-	// GetFailureReason returns the reason the payment failed.
-	GetFailureReason() *channeldb.FailureReason
-
 	// AllowMoreAttempts is used to decide whether we can safely attempt
 	// more HTLCs for a given payment state. Return an error if the payment
 	// is in an unexpected state.
 	AllowMoreAttempts() (bool, error)
+
+	// TerminalInfo returns the settled HTLC attempt or the payment's
+	// failure reason.
+	TerminalInfo() (*channeldb.HTLCAttempt, *channeldb.FailureReason)
 }
 
 // ControlTower tracks all outgoing payments made, whose primary purpose is to