ep-engine: address potential data races in the warmup code

WARNING: ThreadSanitizer: data race (pid=7023)

  Write of size 8 at 0x7d240000d5e0 by thread T7:

    #0 Warmup::checkForAccessLog() /home/daver/repos/couchbase/server/ep-engine/src/warmup.cc:590 (ep.so+0x0000002d1ffc)

    #1 WarmupCheckforAccessLog::run() /home/daver/repos/couchbase/server/ep-engine/src/warmup.h:303 (ep.so+0x0000002e2bfb)

    #2 ExecutorThread::run() /home/daver/repos/couchbase/server/ep-engine/src/executorthread.cc:106 (ep.so+0x0000001e34f9)

    #3 launch_executor_thread(void*) /home/daver/repos/couchbase/server/ep-engine/src/executorthread.cc:34 (ep.so+0x0000001e2b7a)

    #4 platform_thread_wrap /home/daver/repos/couchbase/server/platform/src/cb_pthreads.c:19 (libplatform.so.0.1.0+0x0000000035dc)

  Previous read of size 8 at 0x7d240000d5e0 by main thread (mutexes: write M699180732592992152):

    #0 Warmup::addStats(void (*)(char const*, unsigned short, char const*, unsigned int, void const*), void const*) const /home/daver/repos/couchbase/server/ep-engine/src/warmup.cc:893 (ep.so+0x0000002d6086)

    #1 EventuallyPersistentEngine::getStats(void const*, char const*, int, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /home/daver/repos/couchbase/server/ep-engine/src/ep_engine.cc:4422 (ep.so+0x000000151813)

    #2 EvpGetStats(engine_interface*, void const*, char const*, int, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /home/daver/repos/couchbase/server/ep-engine/src/ep_engine.cc:214 (ep.so+0x0000001367b2)

    #3 mock_get_stats /home/daver/repos/couchbase/server/memcached/programs/engine_testapp/engine_testapp.c:194 (engine_testapp+0x0000004bde63)

    #4 wait_for_warmup_complete(engine_interface*, engine_interface_v1*) /home/daver/repos/couchbase/server/ep-engine/tests/ep_test_apis.cc:898 (ep_testsuite.so+0x0000000ead95)

    #5 test_setup(engine_interface*, engine_interface_v1*) /home/daver/repos/couchbase/server/ep-engine/tests/ep_testsuite.cc:168 (ep_testsuite.so+0x0000000237d3)

    #6 execute_test /home/daver/repos/couchbase/server/memcached/programs/engine_testapp/engine_testapp.c:1037 (engine_testapp+0x0000004ba82b)

    #7 main /home/daver/repos/couchbase/server/memcached/programs/engine_testapp/engine_testapp.c:1296 (engine_testapp+0x0000004b8861)