[Cache] Unathorized clients should be blocked before accessing cache

[vankoven]

When a new request is received it's forwarded to cache first:

tempesta/tempesta_fw/http.c

Line 2373 in 195d85e

if (tfw_cache_process(req, NULL, tfw_http_req_cache_cb)) {

and only after that sticky cookie is checked:

tempesta/tempesta_fw/http.c

Lines 1966 to 1970 in 195d85e

	r = tfw_http_sess_obtain(req);
	if (r < 0)
	goto send_500;
	if (r > 0) /* Response sent, nothing to do. */
	return;

That leads to unnecessary cache lookup for requests with violated or expired sticky cookie. Early blocking of unauthorized clients will help us to save resources under DDOS attacks.