ep-engine: possible data race with global task's waketime

    WARINING: ThreadSanitizer: data race (pid=7728)

    Write of size 8 at 0x7d140000e8a8 by main thread (mutexes: write M11519, write M11535):

    0 TaskQueue::_wake(SingleThreadedRCPtr<GlobalTask>&) /home/abhinav/couchbase/ep-engine/src/taskqueue.cc:272 (ep.so+0x000000142b59)

    1 TaskQueue::wake(SingleThreadedRCPtr<GlobalTask>&) /home/abhinav/couchbase/ep-engine/src/taskqueue.cc:299 (ep.so+0x00000014382e)

    2 ExecutorPool::_stopTaskGroup(unsigned long, task_type_t, bool) /home/abhinav/couchbase/ep-engine/src/executorpool.cc:568 (ep.so+0x0000000f2f46)

    3 ExecutorPool::stopTaskGroup(unsigned long, task_type_t, bool) /home/abhinav/couchbase/ep-engine/src/executorpool.cc:585 (ep.so+0x0000000f31ee)

    4 ~EventuallyPersistentStore /home/abhinav/couchbase/ep-engine/src/ep.cc:468 (ep.so+0x0000000830f6)

    5 ~EventuallyPersistentEngine /home/abhinav/couchbase/ep-engine/src/ep_engine.cc:6326 (ep.so+0x0000000d42ba)

    6 EvpDestroy(engine_interface*, bool) /home/abhinav/couchbase/ep-engine/src/ep_engine.cc:141 (ep.so+0x0000000b3fbc)

    7 mock_destroy(engine_interface*, bool) /home/abhinav/couchbase/memcached/programs/engine_testapp/engine_testapp.cc:98 (engine_testapp+0x0000000ba027)

    8 destroy_bucket(engine_interface*, engine_interface_v1*, bool) /home/abhinav/couchbase/memcached/programs/engine_testapp/engine_testapp.cc:995 (engine_testapp+0x0000000b952e)

    9 __libc_start_main /build/buildd/eglibc-2.19/csu/libc-start.c:287 (libc.so.6+0x000000021ec4)

    Previous write of size 8 at 0x7d140000e8a8 by thread T10:

    0 GlobalTask::snooze(double) /home/abhinav/couchbase/ep-engine/src/tasks.cc:56 (ep.so+0x00000013b6fa)

    1 ConnManager::run() /home/abhinav/couchbase/ep-engine/src/connmap.cc:151 (ep.so+0x00000005032e)

    2 ExecutorThread::run() /home/abhinav/couchbase/ep-engine/src/executorthread.cc:112 (ep.so+0x0000000f86da)

    3 launch_executor_thread(void*) /home/abhinav/couchbase/ep-engine/src/executorthread.cc:33 (ep.so+0x0000000f82e5)

    4 platform_thread_wrap /home/abhinav/couchbase/platform/src/cb_pthreads.c:23 (libplatform.so.0.1.0+0x000000003d31)