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  Hmm, this is actually a bit tricky because it means we're going to have to call http.ListenAndServe again and close the old server, which apparently isn't really possible (https://code.google.com/p/go/issues/detail?id=4674).

Do you have any ideas how to get around this or do we not want to support updating that part of the config?