message: add destructor of cache_struct according to sonar
diff: @@ -172,6 +172,7 @@ private:
  // TODO: solve race conditions when multithreading
  mutable struct cache_struct {
  cache_struct() = default;
  + ~cache_struct() = default;
  cache_struct(const cache_struct &other) {}
  cache_struct &operator=(const cache_struct &other) { return *this; }
  