routing/payment_lifecycle+channeldb: enable multi shard send

This commit finally enables MP payments within the payment lifecycle
(used for SendPayment). This is done by letting the loop launch shards
as long as there is value remaining to send, inspecting the outcomes for
the sent shards when the full payment amount has been filled.

The method channeldb.MPPayment.SentAmt() is added to easily look up how
much value we have sent for the payment.

channeldb/mp_payment.go

@@ -144,6 +144,24 @@ func (m *MPPayment) TerminalInfo() (*HTLCSettleInfo, *FailureReason) {
 	return nil, m.FailureReason
 }
 
+// SentAmt returns the sum of sent amount and fees for HTLCs that are either
+// settled or still in flight.
+func (m *MPPayment) SentAmt() (lnwire.MilliSatoshi, lnwire.MilliSatoshi) {
+	var sent, fees lnwire.MilliSatoshi
+	for _, h := range m.HTLCs {
+		if h.Failure != nil {
+			continue
+		}
+
+		// The attempt was not failed, meaning the amount was
+		// potentially sent to the receiver.
+		sent += h.Route.ReceiverAmt()
+		fees += h.Route.TotalFees()
+	}
+
+	return sent, fees
+}
+
 // InFlightHTLCs returns the HTLCs that are still in-flight, meaning they have
 // not been settled or failed.
 func (m *MPPayment) InFlightHTLCs() []HTLCAttempt {