channeldb: make database logic MPP compatible

This commit redefines how the control tower handles shard and payment
level settles and failures. We now consider the payment in flight as
long it has active shards, or it has no active shards but has not
reached a terminal condition (settle of one of the shards, or a payment
level failure has been encountered).

We also make it possible to settle/fail shards regardless of the payment
level status (since we must allow late shards recording their status
even though we have already settled/failed the payment).

Finally, we make it possible to Fail the payment when it is already
failed. This is to allow multiple concurrent shards that reach terminal
errors to mark the payment failed, without havinng to synchronize.

channeldb/mp_payment.go

@@ -131,6 +131,19 @@ type MPPayment struct {
 	Status PaymentStatus
 }
 
+// TerminalInfo returns any HTLC settle info recorded. If no settle info is
+// recorded, any payment level failure will be returned. If neither a settle
+// nor a failure is recorded, both return values will be nil.
+func (m *MPPayment) TerminalInfo() (*HTLCSettleInfo, *FailureReason) {
+	for _, h := range m.HTLCs {
+		if h.Settle != nil {
+			return h.Settle, nil
+		}
+	}
+
+	return nil, m.FailureReason
+}
+
 // InFlightHTLCs returns the HTLCs that are still in-flight, meaning they have
 // not been settled or failed.
 func (m *MPPayment) InFlightHTLCs() []HTLCAttempt {