responder: fix domain lookup refresh timeout
    
    Currently the timeout to check for new domains is reset whenever there
    is a request with an unknown domain name. If those requests happen more
    then once a minute (the default timeout) the timeout is always reset
    before it can expire and as a result the domain list is never refreshed.
    
    If SSSD starts offline with an empty domain list the NSS responder might
    never be able to resolve sub-domain users or groups even if the backend
    has switched to the online state and already refreshed the domain list.
    
    Related to https://pagure.io/SSSD/sssd/issue/3967
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit c01364341f22262ebc2012b102be872ec3f8bdf4)