roles/nginx: Add fastcgi caching

Bypasses caching for logged in users (right now only for sessions
where the "wordpress_logged_in" cookie is set. Doubles the trans-
actions per second as measured by siege:

    $ siege -d1 -t1M -c50 https://mjanja.ch

Signed-off-by: Alan Orth <alan.orth@gmail.com>

roles/nginx/files/fastcgi_cache

@@ -0,0 +1,4 @@
+fastcgi_cache_key $scheme$host$request_uri;
+# According to the docs, a 1m key zone can store ~8,000 keys, so 10m should
+# be enough. See: http://nginx.org/en/docs/http/ngx_http_fastcgi_module.html
+fastcgi_cache_path /var/cache/nginx/cached/fastcgi levels=2:2 keys_zone=global:10m inactive=60m max_size=1G;

roles/nginx/files/nginx.conf

@@ -37,5 +37,6 @@ http {
 
     client_max_body_size 12m;
 
+    include fastcgi_cache;
     include /etc/nginx/conf.d/*.conf;
 }