rate-limit by default (to a high limit anyway) and queue requests to same key

[kapouer]

First it protects upstream in case of several kinds of ddos,
and second it improves performance in case of simultaneous requests.

Idea for implementation in
openresty/srcache-nginx-module#53

[kapouer]

See also https://medium.com/@sportebois/nginx-rate-limiting-in-a-nutshell-128fe9e0126c#.ilkvq4rh5