Serve stale responses while updating the HTTP cache

Also ensures that we only send one request to upstream when
the cache is invalid.

roles/float-infra-nginx/templates/config/conf.d/proxy.conf

@@ -32,3 +32,9 @@ proxy_cache_min_uses 2;
 # Show our own error pages, not the remote ones.
 proxy_intercept_errors on;
 
+# Ensure we only send one upstream request, and allow NGINX to serve
+# stale data while updating.
+proxy_cache_use_stale updating;
+proxy_cache_background_update on;
+proxy_cache_lock on;
+