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
I also wanted to do that, but I couldn't see an easy way to change the handler for a server created with http.ListenAndServe. It seems like there might be a couple ways to do something like that (maybe by creating the server in a different way), but I wasn't sure that changing that was a better option than just modifying the existing handler. What do you think?