message: Make missing-key callback in PS thread-safe.
diff: @@ -571,8 +571,10 @@ void parameter_server<TypeHashKey>::look_up(const TypeHashKey* h_embeddingcolumn
  // Layer 0: Do a sequential lookup. Remember missing keys.
  std::vector<size_t> indices;
  std::vector<size_t> missing;
  + std::mutex missing_guard;
  - MissingKeyCallback record_missing_fn = [&missing](size_t index) -> void {
  + MissingKeyCallback record_missing_fn = [&missing, &missing_guard](size_t index) -> void {
  + std::lock_guard<std::mutex> lock(missing_guard);
  missing.push_back(index);
  };
  