CACHE_REQ: Check the caches first
    
    This patch introduces a new configurable option to define whether the
    responder should query all domains' caches before querying the Data
    Providers.
    
    This new option is called cache_first and, by default, it's disabled,
    meaning that, for each provider, the responder may contact the cache and
    the data provider in the same iteration.
    
    Co-Author: Pavel Březina <pbrezina@redhat.com>
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3001
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>