Merge branch 'branch-0.4.0' into 'branch-0.4.0'

Update lockguard

See merge request megasearch/milvus!565

Former-commit-id: cbb039c1da725a1c8ce0c8d06150749a6157ded7

cpp/src/scheduler/task/SearchTask.cpp

@@ -271,7 +271,7 @@ Status XSearchTask::MergeResult(SearchContext::Id2DistanceMap &distance_src,
         return Status::OK();
     }
 
-    merge_mutex_.lock();
+    std::unique_lock<std::mutex> lock(merge_mutex_);
     if (distance_target.empty()) {
         distance_target.swap(distance_src);
         return Status::OK();
@@ -331,7 +331,6 @@ Status XSearchTask::MergeResult(SearchContext::Id2DistanceMap &distance_src,
     }
 
     distance_target.swap(distance_merged);
-    merge_mutex_.unlock();
 
     return Status::OK();
 }