He plugin: Statistics collector thread may be blocked forever.

Version:
master (49cc69e154249c8cd0942905fa321e6b4e0fb46d)

Steps to reproduce:
1. Run controller and openflowplugin-he.
2. Start mininet, and then stop it.
3. Repeat step 2 again and again.

Step 2 may record the following log.

2016-05-19 19:57:42,171 | WARN | Pool-8-worker-11 | StatRpcMsgManagerImpl | 179 - org.opendaylight.openflowplugin.applications.statistics-manager - 0.3.0.SNAPSHOT | Response Registration for Statistics RPC call fail!
org.opendaylight.controller.md.sal.dom.api.DOMRpcImplementationNotAvailableException: No local or remote implementation available for rpc AbsoluteSchemaPath

{path=[(urn:opendaylight:group:statistics?revision=2013-11-11)get-all-group-statistics]}

at org.opendaylight.controller.remote.rpc.RemoteRpcImplementation$1.onComplete(RemoteRpcImplementation.java:66)[161:org.opendaylight.controller.sal-remoterpc-connector:1.4.0.SNAPSHOT]
at org.opendaylight.controller.remote.rpc.RemoteRpcImplementation$1.onComplete(RemoteRpcImplementation.java:56)[161:org.opendaylight.controller.sal-remoterpc-connector:1.4.0.SNAPSHOT]
at akka.dispatch.OnComplete.internal(Future.scala:259)[146:com.typesafe.akka.actor:2.4.4]
at akka.dispatch.OnComplete.internal(Future.scala:256)[146:com.typesafe.akka.actor:2.4.4]
at akka.dispatch.japi$CallbackBridge.apply(Future.scala:186)[146:com.typesafe.akka.actor:2.4.4]
at akka.dispatch.japi$CallbackBridge.apply(Future.scala:183)[146:com.typesafe.akka.actor:2.4.4]
at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:32)[142:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.impl.ExecutionContextImpl$AdaptedForkJoinTask.exec(ExecutionContextImpl.scala:121)[142:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)[142:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)[142:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)[142:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)[142:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]

If that warning is logged, statistics collector thread is being blocked and no one wakes it up.

"odl-stat-collector-1-thread-0" #936 prio=5 os_prio=0 tid=0x00007f68ac054000 nid=0x5914 waiting on condition [0x00007f67dbb4f000]
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)

• parking to wait for <0x000000008bf40fd8> (a com.google.common.util.concurrent.AbstractFuture$Sync)
  at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:997)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1304)
  at com.google.common.util.concurrent.AbstractFuture$Sync.get(AbstractFuture.java:285)
  at com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:116)
  at org.opendaylight.openflowplugin.applications.statistics.manager.impl.StatPermCollectorImpl.collectStatCrossNetwork(StatPermCollectorImpl.java:327)
  at org.opendaylight.openflowplugin.applications.statistics.manager.impl.StatPermCollectorImpl.run(StatPermCollectorImpl.java:248)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
  at java.lang.Thread.run(Thread.java:745)