- 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