Fix race in use of gmtime()

As identified by ThreadSanitizer:

    WARNING: ThreadSanitizer: data race (pid=17259)
      Write of size 8 at 0x7fec86e44de0 by main thread (mutexes: write M1161):
	#0 gmtime ??:0 (libtsan.so.0+0x000000025135)
	#1 EventuallyPersistentEngine::doEngineStats(void const*, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /repos/couchbase/server/source/ep-engine/src/ep_engine.cc:3369 (ep.so+0x00000010f4be)
	#2 EventuallyPersistentEngine::getStats(void const*, char const*, int, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /repos/couchbase/server/source/ep-engine/src/ep_engine.cc:4339 (ep.so+0x000000113c35)
	#3 EvpGetStats /repos/couchbase/server/source/ep-engine/src/ep_engine.cc:217 (ep.so+0x000000102b14)
	#4 mock_get_stats /repos/couchbase/server/source/memcached/programs/engine_testapp/engine_testapp.c:195 (exe+0x0000000026de)
	#5 get_int_stat(engine_interface*, engine_interface_v1*, char const*, char const*) /repos/couchbase/server/source/ep-engine/tests/ep_test_apis.cc:799 (ep_testsuite.so+0x0000000832d8)
	#6 wait_for_stat_change(engine_interface*, engine_interface_v1*, char const*, int, char const*) /repos/couchbase/server/source/ep-engine/tests/ep_test_apis.cc:846 (ep_testsuite.so+0x0000000838d6)
	#7 test_setup /repos/couchbase/server/source/ep-engine/tests/ep_testsuite.cc:178 (ep_testsuite.so+0x00000001ee69)
	#8 execute_test /repos/couchbase/server/source/memcached/programs/engine_testapp/engine_testapp.c:1050 (exe+0x000000005970)
	#9 main /repos/couchbase/server/source/memcached/programs/engine_testapp/engine_testapp.c:1313 (exe+0x000000006606)

      Previous write of size 8 at 0x7fec86e44de0 by thread T7:
	#0 gmtime ??:0 (libtsan.so.0+0x000000025135)
	#1 EventuallyPersistentEngine::doEngineStats(void const*, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /repos/couchbase/server/source/ep-engine/src/ep_engine.cc:3369 (ep.so+0x00000010f4be)
	#2 EventuallyPersistentEngine::getStats(void const*, char const*, int, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /repos/couchbase/server/source/ep-engine/src/ep_engine.cc:4339 (ep.so+0x000000113c35)
	#3 EventuallyPersistentStore::snapshotStats() /repos/couchbase/server/source/ep-engine/src/ep.cc:1465 (ep.so+0x0000000e150e)
	#4 StatSnap::run() /repos/couchbase/server/source/ep-engine/src/tasks.cc:79 (ep.so+0x000000174db2)
	#5 ExecutorThread::run() /repos/couchbase/server/source/ep-engine/src/executorthread.cc:110 (ep.so+0x00000014a0e7)
	#6 launch_executor_thread /repos/couchbase/server/source/ep-engine/src/executorthread.cc:34 (ep.so+0x000000149930)
	#7 platform_thread_wrap /repos/couchbase/server/source/platform/src/cb_pthreads.c:19 (libplatform.so.0.1.0+0x000000002d8b)
	#8 __tsan_write_range ??:0 (libtsan.so.0+0x00000001b1c9)

Switch to gmtime_r which is thread-safe.

Change-Id: Id0773df65f4fc569c0a173b6185b1ef8bd91862d
Reviewed-on: http://review.couchbase.org/43312
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>