Address data race seen with ep-engine's startup time

13:05:37 WARNING: ThreadSanitizer: data race (pid=214405)

13:05:37   Read of size 8 at 0x7d740001f3d0 by thread T9:

13:05:37     #0 void STATWRITER_NAMESPACE::add_casted_stat<long>(char const*, long const&, void (*)(char const*, unsigned short, char const*, unsigned int, void const*), void const*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/statwriter.h:47 (ep-engine_stream_test+0x0000005541d9)

13:05:37     #1 EventuallyPersistentEngine::doEngineStats(void const*, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/ep_engine.cc:3385 (ep-engine_stream_test+0x000000662bb9)

13:05:37     #2 EventuallyPersistentEngine::getStats(void const*, char const*, int, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/ep_engine.cc:4360 (ep-engine_stream_test+0x00000066bb4a)

13:05:37     #3 EventuallyPersistentStore::snapshotStats() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/ep.cc:1379 (ep-engine_stream_test+0x000000606634)

13:05:37     #4 StatSnap::run() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/tasks.cc:80 (ep-engine_stream_test+0x000000766eb3)

13:05:37     #5 ExecutorThread::run() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/executorthread.cc:109 (ep-engine_stream_test+0x0000006df127)

13:05:37     #6 launch_executor_thread(void*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/executorthread.cc:34 (ep-engine_stream_test+0x0000006de75a)

13:05:37     #7 platform_thread_wrap /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/platform/src/cb_pthreads.c (libplatform.so.0.1.0+0x00000000377c)

13:05:37 

13:05:37   Previous write of size 8 at 0x7d740001f3d0 by main thread:

13:05:37     #0 EventuallyPersistentEngine::initialize(char const*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/ep_engine.cc:2039 (ep-engine_stream_test+0x0000006578ab)

13:05:37     #1 test_mb17766(std::string const&) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/tests/module_tests/stream_test.cc:79 (ep-engine_stream_test+0x00000050d46e)

13:05:37     #2 main /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/tests/module_tests/stream_test.cc:155 (ep-engine_stream_test+0x00000050cee3)

13:05:37 

13:05:37   Location is heap block of size 2520 at 0x7d740001ea00 allocated by main thread:

13:05:37     #0 operator new(unsigned long) <null> (ep-engine_stream_test+0x0000004a763d)

13:05:37     #1 create_instance /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/ep_engine.cc:1735 (ep-engine_stream_test+0x00000064fc68)

13:05:37     #2 test_mb17766(std::string const&) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/tests/module_tests/stream_test.cc:67 (ep-engine_stream_test+0x00000050d24e)

13:05:37     #3 main /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/tests/module_tests/stream_test.cc:155 (ep-engine_stream_test+0x00000050cee3)