Transaction leak in org.opendaylight.netvirt.natservice.internal.NatRouterInterfaceListener

Seen in scale testing:

{{ 890x transactions opened here, which are not closed:
(...)
org.opendaylight.controller.md.sal.binding.impl.BindingDOMDataBrokerAdapter.newWriteOnlyTransaction(BindingDOMDataBrokerAdapter.java:69)
(...)
org.opendaylight.netvirt.natservice.internal.NatRouterInterfaceListener.add(NatRouterInterfaceListener.java:80)
org.opendaylight.netvirt.natservice.internal.NatRouterInterfaceListener.add(NatRouterInterfaceListener.java:30)
org.opendaylight.genius.datastoreutils.AsyncDataTreeChangeListenerBase$DataTreeChangeHandler.run(AsyncDataTreeChangeListenerBase.java:172)
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
java.lang.Thread.run(Thread.java:748)}}