Fixed bugs in reviewed code

Author: YuCai18

src/storage/src/redis.cc

@@ -208,8 +208,7 @@ void Redis::GetBigKeyStatistics(std::vector<BigKeyInfo>* bigkeys) {
   if (!bigkeys) {
     return;
   }
-  
-  std::lock_guard<std::mutex> lock(big_keys_mutex_);
+  std::shared_lock lock(big_keys_mutex_);
   for (const auto& kv : big_keys_info_map_) {
     BigKeyInfo info = kv.second;
     info.key = kv.first;