message: mavsdk_server: fix crash on disconnect
  This should prevent the mutex going out of scope.
diff: @@ -21,14 +21,11 @@ public:
  const rpc::core::SubscribeConnectionStateRequest* /* request */,
  grpc::ServerWriter<rpc::core::ConnectionStateResponse>* writer) override
  {
  - std::mutex connection_state_mutex{};
  -
  - _mavsdk.subscribe_on_new_system([this, writer, &connection_state_mutex]() {
  - publish_system_state(writer, connection_state_mutex);
  - });
  + _mavsdk.subscribe_on_new_system(
  + [this, writer]() { publish_system_state(writer, _connection_state_mutex); });
  // Publish the current state on subscribe
  - publish_system_state(writer, connection_state_mutex);
  + publish_system_state(writer, _connection_state_mutex);
  _stop_future.wait();
  return grpc::Status::OK;
  @@ -51,8 +48,8 @@ public:
  private:
  Mavsdk& _mavsdk;
  std::promise<void> _stop_promise;
  -
  std::future<void> _stop_future;
  + std::mutex _connection_state_mutex{};
  static mavsdk::rpc::core::ConnectionStateResponse
  createRpcConnectionStateResponse(const bool is_connected)
  