sss_client: refactor internal timeout handling
    
    This patch adds a timeout option to the internal client calls so that
    the timeout is not hard-coded anymore in the low level poll() calls but
    can be set by the caller with sss_nss_make_request_timeout(). Since the
    old timeout value is not changed by this patch there is no functional
    change expected.
    
    Related to https://pagure.io/SSSD/sssd/issue/2478
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>