- only add an async version. I think sync requests are fundamentally flawed
  because they mess up the order of D-Bus messages. Hence, also don't
  call the function *_async(), like we do for other functions. As there
  is only the async form, it doesn't have a suffix.

- Don't accept a NMConnection as @settings argument, but a GVariant.
  In general, keep the libnm API closer to the D-Bus API and don't hide
  the underlying function with a less powerful form. The user still can
  conveniently call the function with

    nm_remote_connection_update2 (connection,
                                  nm_connection_to_dbus (NM_CONNECTION (connection),
                                                         NM_CONNECTION_SERIALIZE_ALL),
                                  save_to_disk
                                    ? NM_SETTINGS_UPDATE2_FLAG_TO_DISK
                                    : NM_SETTINGS_UPDATE2_FLAG_IN_MEMORY,
                                  NULL,
                                  cancellable,
                                  callback,
                                  user_data);

  I believe the parts of libnm that invoke D-Bus methods, should be
  close to the D-Bus API. Not like nm_remote_connection_commit_changes()
  which has no corresponding D-Bus method.