MB-21325: Fix race in BloomFilter::getNumOfKeysInFilter

MB-21325: Fix race in BloomFilter::getNumOfKeysInFilter

Address race condition identified by ThreadSanitizer on
BloomFilter::keyCounter.

WARNING: ThreadSanitizer: data race (pid=341)
  Write of size 8 at 0x7d140000df80 by thread T11 (mutexes: write M20582, write M20817, write M19369):
    #0 BloomFilter::addKey() ep-engine/src/bloomfilter.cc:119 (ep.so+0x00000002453d)
    #1 VBucket::addToFilter() ep-engine/src/vbucket.cc:482 (ep.so+0x000000149704)
    #2 PersistenceCallback::callback() ep-engine/src/ep.cc:3211 (ep.so+0x0000000b27c1)
    #3 non-virtual thunk to PersistenceCallback::callback(int&) ep-engine/src/ep.cc:3167 (ep.so+0x0000000b36c2)
    #4 CouchKVStore::commitCallback() ep-engine/src/couch-kvstore/couch-kvstore.cc:1898 (ep.so+0x00000017aa51)
    #5 _ZN12CouchKVStore17commit2couchstoreEP8CallbackIJ10KVStatsCtxEE ep-engine/src/couch-kvstore/couch-kvstore.cc:1712 (ep.so+0x00000017208b)
    #6 _ZN12CouchKVStore6commitEP8CallbackIJ10KVStatsCtxEE ep-engine/src/couch-kvstore/couch-kvstore.cc:954 (ep.so+0x000000171c92)
    #7 EventuallyPersistentStore::commit(unsigned short) ep-engine/src/ep.cc:3451 (ep.so+0x0000000a29e6)
    #8 EventuallyPersistentStore::flushVBucket(unsigned short) ep-engine/src/ep.cc:3399 (ep.so+0x0000000a1935)
    #9 Flusher::flushVB() ep-engine/src/flusher.cc:293 (ep.so+0x0000001038ae)
    #10 Flusher::step(GlobalTask*) ep-engine/src/flusher.cc:183 (ep.so+0x000000101f7e)
    #11 FlusherTask::run() ep-engine/src/tasks.cc:138 (ep.so+0x00000013ac42)
    #12 ExecutorThread::run() ep-engine/src/executorthread.cc:115 (ep.so+0x0000000fd53f)
    #13 launch_executor_thread(void*) ep-engine/src/executorthread.cc:33 (ep.so+0x0000000fd095)
    #14 platform_thread_wrap(void*) platform/src/cb_pthreads.cc:55 (libplatform_so.so.0.1.0+0x000000005deb)

  Previous read of size 8 at 0x7d140000df80 by main thread (mutexes: write M15538, write M1828880534441356728):
    #0 BloomFilter::getNumOfKeysInFilter() ep-engine/src/bloomfilter.cc:142 (ep.so+0x0000000246a0)
    #1 VBucket::addStats() ep-engine/src/vbucket.cc:597 (ep.so+0x00000014a527)
    #2 EventuallyPersistentEngine::doVBucketStats() ep-engine/src/ep_engine.cc:3837 (ep.so+0x0000000c69f6)
    #3 EventuallyPersistentEngine::doVBucketStats() ep-engine/src/ep_engine.cc:3861 (ep.so+0x0000000c664a)
    #4 EventuallyPersistentEngine::getStats() ep-engine/src/ep_engine.cc:4774 (ep.so+0x0000000cbbb3)
    #5 EvpGetStats() ep-engine/src/ep_engine.cc:231 (ep.so+0x0000000baafe)
    #6 mock_get_stats() memcached/programs/engine_testapp/engine_testapp.cc:215 (engine_testapp+0x0000004ce40d)
    #7 std::string get_stat<std::string>() ep-engine/tests/ep_test_apis.cc:1140 (ep_testsuite_xdcr.so+0x000000020c34)
    #8 unsigned long get_stat<unsigned long>() ep-engine/tests/ep_test_apis.cc:1179 (ep_testsuite_xdcr.so+0x00000002089b)
    #9 get_ull_stat() ep-engine/tests/ep_test_apis.cc:1174 (ep_testsuite_xdcr.so+0x00000001e73e)
    #10 test_del_with_meta_and_check_drift_stats() ep-engine/tests/ep_testsuite_xdcr.cc:1711 (ep_testsuite_xdcr.so+0x0000000143f1)
    #11 execute_test() memcached/programs/engine_testapp/engine_testapp.cc:1118 (engine_testapp+0x0000004cc997)
    #12 __libc_start_main /build/buildd/eglibc-2.15/csu/libc-start.c:226 (libc.so.6+0x00000002176c)

Change-Id: I4e9a589e9286e72646df515db891c57b143d1fdd
Reviewed-on: http://review.couchbase.org/68803
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Daniel Owen <owend@couchbase.com>