Daemon should support reload config

This change adds support for changing the limit configuration of a running sphinxd.
It does this by listening for the HUP signal, reloading the config file when
that happens, and changing the ratelimiter object.

We aren't worrying about pre-existing buckets that get affected when we reload,
they will expire out.

I also tested this manually with Apache Bench
Hmm, yeah, I think it's reasonable to say that we don't support changing `proxy.listen` without fully restarting the daemon - what do you think?

I think we'd still want to support modifying `proxy.handler` and `proxy.host`, though, since we can modify that without restarting an HTTP server (just by changing the `http.Handler` used by the daemon)