CACHE_REQ: Move cache_req_next_domain() into a new tevent request
    
    This change prepares for the upcoming patch, which will have to support
    more than one iteration over all domains, leaving us with a better
    control of the whole logic around this iteration.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3001
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>