Exit offline mode only if server is available.
    
    This patch adds periodic check to test if
    backend can exit offline mode and only
    marks backend as not offline if server
    for the service is available.
    
    Prior to this patch we marked backend as not
    offline if the offline_timeout was reached
    without checking for the server availability
    and when the next request failed again
    we switched back to the offline mode.
    This caused significant slowdowns in some
    edge cases.
    
    Fixes:
    https://fedorahosted.org/sssd/ticket/2355
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>