DataChangeListener is not notified of DS change occasionally

We have observed that DataChangeListener was not notified of data change occasionally.
This issue happens on the master branch (for Boron development).

When the issue happened, we observed the following exception was caused during DS update.

2016-05-16 15:17:21,371 | ERROR | lt-dispatcher-32 | OneForOneStrategy | 184 - com.typesafe.akka.slf4j - 2.4.4 | This stopwatch is already running.
java.lang.IllegalStateException: This stopwatch is already running.
at com.google.common.base.Preconditions.checkState(Preconditions.java:173)[83:com.google.guava:18.0.0]
at com.google.common.base.Stopwatch.start(Stopwatch.java:160)[83:com.google.guava:18.0.0]
at org.opendaylight.controller.cluster.datastore.DefaultShardDataChangeListenerPublisher.publishChanges(DefaultShardDataChangeListenerPublisher.java:59)[199:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.ShardDataTreeNotificationPublisherActor$PublishNotifications.publish(ShardDataTreeNotificationPublisherActor.java:46)[199:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.ShardDataTreeNotificationPublisherActor$PublishNotifications.access$000(ShardDataTreeNotificationPublisherActor.java:33)[199:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.ShardDataTreeNotificationPublisherActor.handleReceive(ShardDataTreeNotificationPublisherActor.java:25)[199:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.common.actor.AbstractUntypedActor.onReceive(AbstractUntypedActor.java:25)[195:org.opendaylight.controller.sal-clustering-commons:1.4.0.SNAPSHOT]
at akka.actor.UntypedActor$$anonfun$receive$1.applyOrElse(UntypedActor.scala:165)[183:com.typesafe.akka.actor:2.4.4]
at akka.actor.Actor$class.aroundReceive(Actor.scala:482)[183:com.typesafe.akka.actor:2.4.4]
at akka.actor.UntypedActor.aroundReceive(UntypedActor.scala:95)[183:com.typesafe.akka.actor:2.4.4]
at akka.actor.ActorCell.receiveMessage(ActorCell.scala:526)[183:com.typesafe.akka.actor:2.4.4]
at akka.actor.ActorCell.invoke(ActorCell.scala:495)[183:com.typesafe.akka.actor:2.4.4]
at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:257)[183:com.typesafe.akka.actor:2.4.4]
at akka.dispatch.Mailbox.run(Mailbox.scala:224)[183:com.typesafe.akka.actor:2.4.4]
at akka.dispatch.Mailbox.exec(Mailbox.scala:234)[183:com.typesafe.akka.actor:2.4.4]
at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)[179:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)[179:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)[179:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)[179:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]

Due to this issue, IT for VTN Manager on the master branch fails occasionally.

https://jenkins.opendaylight.org/releng/view/vtn/job/vtn-verify-boron/138/
https://jenkins.opendaylight.org/releng/view/vtn/job/vtn-verify-boron/129/