SBUS: Add a time_t pointer to the sbus_connection
    
    The idea of this commit is to provide a way to update the time of the
    last request coming from/going to the responders through sbus.
    
    For now it's not used anywhere as all the functions that will pass their
    time_t to the sbus_connection are currently passing NULL. It will be
    used by follow-up patches.
    
    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>