dbscripts: Prevent concurrent mirrorauth requests

- Set "proxy_cache_lock on" to avoid sending more than one auth request
  to archweb when a cache entry doesn't exist for the credentials.
- Set "proxy_cache_use_stale updating" to prevent nginx sending multiple
  auth requests to archweb while the cache is being refreshed. [1]

[1] http://mailman.nginx.org/pipermail/nginx/2016-January/049734.html

roles/dbscripts/templates/nginx.d.conf.j2

@@ -44,6 +44,10 @@ server {
         proxy_cache             auth_cache;
         proxy_cache_key         $scheme$proxy_host$uri$http_authorization;
 
+        # Minimize the number of requests to archweb
+        proxy_cache_lock        on;
+        proxy_cache_use_stale   updating;
+
         # Authentication to archweb
         proxy_pass https://archlinux.org;
 	proxy_ssl_verify on;