channeldb: validate MPP options when registering attempts

We add validation making sure we are not trying to register MPP shards
for non-MPP payments, and vice versa. We also add validtion of total
sent amount against payment value, and matching MPP options.

We also add methods for copying Route/Hop, since it is useful to use
for modifying the route amount in the test.

routing/route/route.go

@@ -129,6 +129,23 @@ type Hop struct {
 	LegacyPayload bool
 }
 
+// Copy returns a deep copy of the Hop.
+func (h *Hop) Copy() *Hop {
+	c := *h
+
+	if h.MPP != nil {
+		m := *h.MPP
+		c.MPP = &m
+	}
+
+	if h.AMP != nil {
+		a := *h.AMP
+		c.AMP = &a
+	}
+
+	return &c
+}
+
 // PackHopPayload writes to the passed io.Writer, the series of byes that can
 // be placed directly into the per-hop payload (EOB) for this hop. This will
 // include the required routing fields, as well as serializing any of the
@@ -287,6 +304,18 @@ type Route struct {
 	Hops []*Hop
 }
 
+// Copy returns a deep copy of the Route.
+func (r *Route) Copy() *Route {
+	c := *r
+
+	c.Hops = make([]*Hop, len(r.Hops))
+	for i := range r.Hops {
+		c.Hops[i] = r.Hops[i].Copy()
+	}
+
+	return &c
+}
+
 // HopFee returns the fee charged by the route hop indicated by hopIndex.
 func (r *Route) HopFee(hopIndex int) lnwire.MilliSatoshi {
 	var incomingAmt lnwire.MilliSatoshi
@@ -320,6 +349,15 @@ func (r *Route) ReceiverAmt() lnwire.MilliSatoshi {
 	return r.Hops[len(r.Hops)-1].AmtToForward
 }
 
+// FinalHop returns the last hop of the route, or nil if the route is empty.
+func (r *Route) FinalHop() *Hop {
+	if len(r.Hops) == 0 {
+		return nil
+	}
+
+	return r.Hops[len(r.Hops)-1]
+}
+
 // NewRouteFromHops creates a new Route structure from the minimally required
 // information to perform the payment. It infers fee amounts and populates the
 // node, chan and prev/next hop maps.