reservations: check if channel is closing

This commit adds a check to the runtime loop
of the reservation manager to check if the
channel is closing before trying to read
from it.

Author: sputn1ck

instantout/reservation/manager.go

@@ -64,7 +64,12 @@ func (m *Manager) Run(ctx context.Context, height int32) error {
 			log.Debugf("Received block %v", height)
 			currentHeight = height
 
-		case reservationRes := <-ntfnChan:
+		case reservationRes, ok := <-ntfnChan:
+			if !ok {
+				// The channel has been closed, we'll stop the
+				// reservation manager.
+				return nil
+			}
 			log.Debugf("Received reservation %x",
 				reservationRes.ReservationId)
 			_, err := m.newReservation(