SBUS: Add destructor data to sbus_connection
    
    This additions has a very specific reason: unregister a service when
    it's shutdown.
    
    So far, we never had to do this kind of operation because the services
    were started during SSSD's startup when finished when SSSD finished.
    
    Now, with the socket-activation in place the game will be a little bit
    different as the services will have an idle timeout and will be able
    shut themselves down. In order to do it properly the monitor will need
    to "unregister" the service and there's no way to do that without adding
    this destructor data to the sbus_connection structure and introducing a
    new function to access it from the monitor (where we're going to
    set the destructor function to the sbus_connection for the
    socket-activated services).
    
    So far it's not being used anywhere as every function taking it as
    parameter is just receiving NULL, but it will be used in the follow up
    commits, by the monitor.
    
    Related:
    https://fedorahosted.org/sssd/ticket/3245
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>