channeldb: rename PaymentAttemptInfo to HTLCAttemptInfo

To better distinguish payments from HTLCs, we rename the attempt info
struct to HTLCAttemptInfo. We also embed it into the HTLCAttempt struct,
to avoid having to duplicate this information.

The paymentID term is renamed to attemptID.

routing/router.go

@@ -1768,7 +1768,7 @@ func (r *ChannelRouter) SendToRoute(hash lntypes.Hash, route *route.Route) (
 // router will call this method for every payment still in-flight according to
 // the ControlTower.
 func (r *ChannelRouter) sendPayment(
-	existingAttempt *channeldb.PaymentAttemptInfo,
+	existingAttempt *channeldb.HTLCAttemptInfo,
 	payment *LightningPayment, paySession PaymentSession) (
 	[32]byte, *route.Route, error) {