CACHE_REQ: Add cache_req_data_set_bypass_dp()
    
    Similar to cache_req_data_set_bypass_cache()
    cache_req_data_set_bypass_dp() can be used to control how the cache_req
    framework performs the lookup. If cache_req_data_set_bypass_dp() is used
    with 'true' only a cache lookup is performed and no request is send to
    the backend even if no entry was found.
    
    Related to https://pagure.io/SSSD/sssd/issue/2478
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>