ep-perfsuite: data race during get_str_stat()

WARNING: ThreadSanitizer: data race (pid=14722)

  Write of size 8 at 0x7d080005fe80 by main thread:

    #0 operator delete(void*) <null> (engine_testapp+0x000000464a1b)

    #1 stop_persistence(engine_interface*, engine_interface_v1*) /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/basic_string.h:249 (ep_perfsuite.so+0x000000022f31)

    #2 perf_latency(engine_interface*, engine_interface_v1*, char const*, unsigned long) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-watson/ep-engine/tests/ep_perfsuite.cc:301 (ep_perfsuite.so+0x000000009f2d)

    #3 perf_latency_dcp_impact(engine_interface*, engine_interface_v1*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-watson/ep-engine/tests/ep_perfsuite.cc:1007 (ep_perfsuite.so+0x000000009542)

    #4 execute_test(test, char const*, char const*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-watson/memcached/programs/engine_testapp/engine_testapp.cc:1140 (engine_testapp+0x0000004cc7d0)

    #5 __libc_start_main /build/buildd/eglibc-2.15/csu/libc-start.c:226 (libc.so.6+0x00000002176c)

  Previous read of size 8 at 0x7d080005fe80 by thread T19 (mutexes: write M13176):

    #0 get_str_stat(engine_interface*, engine_interface_v1*, char const*, char const*) /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/basic_string.h:725 (ep_perfsuite.so+0x000000023182)

    #1 get_ull_stat(engine_interface*, engine_interface_v1*, char const*, char const*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-watson/ep-engine/tests/ep_test_apis.cc:1155 (ep_perfsuite.so+0x00000002223b)

    #2 perf_dcp_client(Handle_args*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-watson/ep-engine/tests/ep_perfsuite.cc:692 (ep_perfsuite.so+0x00000000ceb9)

    #3 platform_thread_wrap(void*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-watson/platform/src/cb_pthreads.cc:54 (libplatform.so.0.1.0+0x0000000057fb)