message: Mark `ConcurrentMapContainer::get` as thread-safe and const
  Summary: As titled.
diff: @@ -216,7 +216,10 @@ class ConcurrentMapContainer
  return this->get_container(slot).at(key);
  }
  - Value get(const Key& key, Value default_value) {
  + /*
  + * This operation is always thread-safe.
  + */
  + Value get(const Key& key, Value default_value) const {
  size_t slot = Hash()(key) % n_slots;
  boost::lock_guard<boost::mutex> lock(this->get_lock(slot));
  const auto& map = this->get_container(slot);
  