SUDO: convert periodical refreshes to be_ptask
    
    This removes old sudo timer and simplyfies code a lot. It also
    allows to manage offline/online state.
    
    - Full and smart refresh are disabled when offline.
    - Full refresh is run immediately when sssd is back online.
    - Smart refresh is scheduled normally when sssd is back online.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/1943
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>