channeldb+routing: store all payment htlcs

This commit converts the database structure of a payment so that it can
not just store the last htlc attempt, but all attempts that have been
made. This is a preparation for mpp sending.

In addition to that, we now also persist the fail time of an htlc. In a
later commit, the full failure reason will be added as well.

A key change is made to the control tower interface. Previously the
control tower wasn't aware of individual htlc outcomes. The payment
remained in-flight with the latest attempt recorded, but an outcome was
only set when the payment finished. With this commit, the outcome of
every htlc is expected by the control tower and recorded in the
database.

Co-authored-by: Johan T. Halseth <johanth@gmail.com>

routing/router.go

@@ -541,7 +541,14 @@ func (r *ChannelRouter) Start() error {
 				PaymentHash: payment.Info.PaymentHash,
 			}
 
-			_, _, err := r.sendPayment(payment.Attempt, lPayment, paySession)
+			// TODO(joostjager): For mpp, possibly relaunch multiple
+			// in-flight htlcs here.
+			var attempt *channeldb.HTLCAttemptInfo
+			if len(payment.Attempts) > 0 {
+				attempt = &payment.Attempts[0]
+			}
+
+			_, _, err := r.sendPayment(attempt, lPayment, paySession)
 			if err != nil {
 				log.Errorf("Resuming payment with hash %v "+
 					"failed: %v.", payment.Info.PaymentHash, err)