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
Out of curiosity, why are we modifying an existing `SphinxHandler` instead of putting in a new `http.Handler`?

If we put in a new `http.Handler`, then the HTTP handling code wouldn't need to support any new functionality, and this could all be handled by the `Daemon` alone.