Avoid crashing when notified of updated term during shutdown
Avoid crashing when notified of updated term during shutdown

This fixes the following CHECK failure:

F0513 21:44:05.646955 15945 raft_consensus.cc:444] Check failed: _s.ok() Bad status: Illegal state: Unable to lock ReplicaState for config change: State = 2
*** Check failure stack trace: ***
    @     0x7fcabb82e38d  google::LogMessage::Fail()
    @     0x7fcabb83022d  google::LogMessage::SendToLog()
    @     0x7fcabb82df7c  google::LogMessage::Flush()
    @     0x7fcabb830b4e  google::LogMessageFatal::~LogMessageFatal()
    @     0x7fcab7c79f1d  kudu::consensus::RaftConsensus::NotifyTermChange()
    @     0x7fcab7c12eb0  kudu::consensus::PeerMessageQueue::NotifyObserversOfTermChangeTask()
    @     0x7fcab7c1c661  kudu::internal::InvokeHelper<>::MakeItSo()
    @     0x7fcab7c1c3ab  kudu::internal::Invoker<>::Run()
    @     0x7fcab7c4211c  boost::_bi::bind_t<>::operator()()
    @     0x7fcab4993fbb  boost::function0<>::operator()()
    @     0x7fcab3783df5  kudu::ThreadPool::DispatchThread()
    @     0x7fcab3786dcc  boost::_bi::bind_t<>::operator()()
    @     0x7fcab4993fbb  boost::function0<>::operator()()
    @     0x7fcab376ee68  kudu::Thread::SuperviseThread()
    @     0x7fcab60c3182  start_thread

Change-Id: Ied6a178802053c495cfa5b6cf309ca938c538739
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/6646
Tested-by: jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>

EvilMcJerkface/kudu,cloudera/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,helifu/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,andrwng/kudu,andrwng/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,InspurUSA/kudu,cloudera/kudu,cloudera/kudu,InspurUSA/kudu,andrwng/kudu,andrwng/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu