manager: Keep volatile/external connections while referenced by async_op_lst

If an NMSettingsConnection with the VOLATILE or EXTERNAL flags is created and passed to nm_manager_activate_connection, it's immediately scheduled for deletion in an idle callback and will likely be deleted before the authorization step in nm_manager_activate_connection finishes and the connection will be aborted. This is because there's no NMActiveConnection in priv->active_connection_lst_head referencing it until _internal_activate_device().

In this commit I change active_connection_find_by_connection to also look for connections in priv->async_op_lst_head.

The reason I want this to work is because I'm possibly misusing nm_manager_activate_connection -- see !670 (closed) -- although I can't see a reason passing NMSettingsConnections with those flags shouldn't be supported by nm_manager_activate_connection.