routing/router: persist payment state machine

This commit makes the router use the ControlTower to drive the payment
life cycle state machine, to keep track of active payments across
restarts.  This lets the router resume payments on startup, such that
their final results can be handled and stored when ready.

channeldb/payments.go

@@ -573,6 +573,48 @@ func fetchPayment(bucket *bbolt.Bucket) (*Payment, error) {
 	return p, nil
 }
 
+// DeletePayments deletes all completed and failed payments from the DB.
+func (db *DB) DeletePayments() error {
+	return db.Update(func(tx *bbolt.Tx) error {
+		payments := tx.Bucket(paymentsRootBucket)
+		if payments == nil {
+			return nil
+		}
+
+		var deleteBuckets [][]byte
+		err := payments.ForEach(func(k, _ []byte) error {
+			bucket := payments.Bucket(k)
+			if bucket == nil {
+				// We only expect sub-buckets to be found in
+				// this top-level bucket.
+				return fmt.Errorf("non bucket element in " +
+					"payments bucket")
+			}
+
+			// If the status is InFlight, we cannot safely delete
+			// the payment information, so we return early.
+			paymentStatus := fetchPaymentStatus(bucket)
+			if paymentStatus == StatusInFlight {
+				return nil
+			}
+
+			deleteBuckets = append(deleteBuckets, k)
+			return nil
+		})
+		if err != nil {
+			return err
+		}
+
+		for _, k := range deleteBuckets {
+			if err := payments.DeleteBucket(k); err != nil {
+				return err
+			}
+		}
+
+		return nil
+	})
+}
+
 func serializePaymentCreationInfo(w io.Writer, c *PaymentCreationInfo) error {
 	var scratch [8]byte