Fix race in ConnNotifier.task access

Race identified by ThreadSanitizer:

      Read of size 8 at 0x7d08000c1430 by thread T10:
        #0 ConnNotifier::notifyConnections() /home/vagrant/couchbase-server/ep-engine/src/connmap.cc:127 (ep.so+0x0000002537e8)
        #1 ConnNotifierCallback::run() /home/vagrant/couchbase-server/ep-engine/src/connmap.cc:80 (ep.so+0x000000277e8b)
        #2 ExecutorThread::run() /home/vagrant/couchbase-server/ep-engine/src/executorthread.cc:110 (ep.so+0x0000002163af)
        #3 launch_executor_thread(void*) /home/vagrant/couchbase-server/ep-engine/src/executorthread.cc:34 (ep.so+0x0000002158ba)
        #4 platform_thread_wrap /home/vagrant/couchbase-server/platform/src/cb_pthreads.c:19 (libplatform.so.0.1.0+0x0000000033f4)

      Previous write of size 8 at 0x7d08000c1430 by main thread:
        #0 ConnNotifier::start() /home/vagrant/couchbase-server/ep-engine/src/connmap.cc:99 (ep.so+0x00000025336f)
        #1 ConnMap::initialize(conn_notifier_type) /home/vagrant/couchbase-server/ep-engine/src/connmap.cc:194 (ep.so+0x0000002544a6)
        #2 EventuallyPersistentEngine::initialize(char const*) /home/vagrant/couchbase-server/ep-engine/src/ep_engine.cc:2060 (ep.so+0x000000179b4a)
        #3 EvpInitialize(engine_interface*, char const*) /home/vagrant/couchbase-server/ep-engine/src/ep_engine.cc:135 (ep.so+0x000000173675)
        #4 init_engine /home/vagrant/couchbase-server/memcached/utilities/engine_loader.c:116 (libmcd_util.so.1.0.0+0x000000003fac)
        #5 start_your_engines /home/vagrant/couchbase-server/memcached/programs/engine_testapp/engine_testapp.c:913 (exe+0x0000000a2fb5)
        #6 execute_test /home/vagrant/couchbase-server/memcached/programs/engine_testapp/engine_testapp.c:1048 (exe+0x0000000a3d29)
        #7 main /home/vagrant/couchbase-server/memcached/programs/engine_testapp/engine_testapp.c:1313 (exe+0x0000000a1d84)

Change-Id: I16cfdff1ea363bbb07a62a92f09f829483276b3d
Reviewed-on: http://review.couchbase.org/43318
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>