Added support for annotation session-cookie-change-on-failure

1. Session cookie is updated on previous attempt failure when `session-cookie-change-on-failure = true` (default value is `false`).
2. Added tests to check both cases.
3. Updated docs.

Co-Authored-By: Vladimir Grishin <yadolov@users.noreply.github.com>

rootfs/etc/nginx/lua/util/split.lua

@@ -16,6 +16,12 @@ function _M.get_first_value(var)
   return t[1]
 end
 
+function _M.get_last_value(var)
+  local t = _M.split_upstream_var(var) or {}
+  if #t == 0 then return nil end
+  return t[#t]
+end
+
 -- http://nginx.org/en/docs/http/ngx_http_upstream_module.html#example
 -- CAVEAT: nginx is giving out : instead of , so the docs are wrong
 -- 127.0.0.1:26157 : 127.0.0.1:26157 , ngx.var.upstream_addr