Add rate limiting to nginx.

The only caveat is that "One megabyte zone can keep about 16 thousand
64-byte states. If the zone storage is exhausted, the server will return
the 503 (Service Temporarily Unavailable) error to all further
requests."
  -- nginx documentation

conf/nginx-nossl.conf

@@ -5,6 +5,8 @@ include root-base.conf;
 http {
     include http-base.conf;
 
+    include rate-limiting.conf;
+
     server {
         include server-base.conf;