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
@azylman Ah, I see that only would be a problem if someone changed the proxy.Listen port. We could change the proxy.Host. I'm not sure, what do you think?