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/control_tower_test.go

@@ -324,7 +324,7 @@ func genInfo() (*channeldb.PaymentCreationInfo, *channeldb.HTLCAttemptInfo,
 	rhash := sha256.Sum256(preimage[:])
 	return &channeldb.PaymentCreationInfo{
 			PaymentHash:    rhash,
-			Value:          1,
+			Value:          testRoute.ReceiverAmt(),
 			CreationTime:   time.Unix(time.Now().Unix(), 0),
 			PaymentRequest: []byte("hola"),
 		},