Fix race in Flusher._state

Fix race identified by ThreadSanitizer:

    WARNING: ThreadSanitizer: data race (pid=10664)
      Read of size 4 at 0x7d4400009888 by main thread (mutexes: write M18321):
        #0 Flusher::stateName() const /home/vagrant/couchbase-server/ep-engine/src/flusher.cc:119 (ep.so+0x0000001fc22e)
        #1 EventuallyPersistentEngine::doEngineStats(void const*, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /home/vagrant/couchbase-server/ep-engine/src/ep_engine.cc:3025 (ep.so+0x000000180649)
        #2 EventuallyPersistentEngine::getStats(void const*, char const*, int, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /home/vagrant/couchbase-server/ep-engine/src/ep_engine.cc:4374 (ep.so+0x00000018d960)
        #3 EvpGetStats(engine_interface*, void const*, char const*, int, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /home/vagrant/couchbase-server/ep-engine/src/ep_engine.cc:214 (ep.so+0x000000174062)
        #4 mock_get_stats /home/vagrant/couchbase-server/memcached/programs/engine_testapp/engine_testapp.c:196 (exe+0x0000000a741d)
        #5 get_int_stat(engine_interface*, engine_interface_v1*, char const*, char const*) /home/vagrant/couchbase-server/ep-engine/tests/ep_test_apis.cc:799 (ep_testsuite.so+0x0000000dae46)
        #6 verify_curr_items(engine_interface*, engine_interface_v1*, int, char const*) /home/vagrant/couchbase-server/ep-engine/tests/ep_test_apis.cc:834 (ep_testsuite.so+0x0000000e1d31)
        #7 test_dcp_producer_stream_req_disk(engine_interface*, engine_interface_v1*) /home/vagrant/couchbase-server/ep-engine/tests/ep_testsuite.cc:3589 (ep_testsuite.so+0x000000094c9a)
        #8 execute_test /home/vagrant/couchbase-server/memcached/programs/engine_testapp/engine_testapp.c:1055 (exe+0x0000000a3e83)
        #9 main /home/vagrant/couchbase-server/memcached/programs/engine_testapp/engine_testapp.c:1313 (exe+0x0000000a1d84)

      Previous write of size 4 at 0x7d4400009888 by thread T5:
        #0 Flusher::transition_state(flusher_state) /home/vagrant/couchbase-server/ep-engine/src/flusher.cc:114 (ep.so+0x0000001fb7df)
        #1 Flusher::step(GlobalTask*) /home/vagrant/couchbase-server/ep-engine/src/flusher.cc:167 (ep.so+0x0000001fc9d1)
        #2 FlusherTask::run() /home/vagrant/couchbase-server/ep-engine/src/tasks.cc:44 (ep.so+0x00000027870e)
        #3 ExecutorThread::run() /home/vagrant/couchbase-server/ep-engine/src/executorthread.cc:110 (ep.so+0x0000002160ff)
        #4 launch_executor_thread(void*) /home/vagrant/couchbase-server/ep-engine/src/executorthread.cc:34 (ep.so+0x00000021560a)
        #5 platform_thread_wrap /home/vagrant/couchbase-server/platform/src/cb_pthreads.c:19 (libplatform.so.0.1.0+0x0000000033f4)

Change-Id: Iaeb60efdc1032de7ba344e04cce454cc1d876d40
Reviewed-on: http://review.couchbase.org/43315
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>