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.

channeldb/mp_payment.go

@@ -219,10 +219,10 @@ func (m *MPPayment) Terminated() bool {
 // TerminalInfo returns any HTLC settle info recorded. If no settle info is
 // recorded, any payment level failure will be returned. If neither a settle
 // nor a failure is recorded, both return values will be nil.
-func (m *MPPayment) TerminalInfo() (*HTLCSettleInfo, *FailureReason) {
+func (m *MPPayment) TerminalInfo() (*HTLCAttempt, *FailureReason) {
 	for _, h := range m.HTLCs {
 		if h.Settle != nil {
-			return h.Settle, nil
+			return &h, nil
 		}
 	}
 
@@ -464,11 +464,6 @@ func (m *MPPayment) GetHTLCs() []HTLCAttempt {
 	return m.HTLCs
 }
 
-// GetFailureReason returns the failure reason.
-func (m *MPPayment) GetFailureReason() *FailureReason {
-	return m.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.