SIGHUP now reloads Haproxy config.

Defaults/globals/etc. service_config is currently not reloaded.

There is a race condition in Haproxy::State: if force_restart is
set and an updated state is received there will likely be two iterations of the worker thread loop, leading to an unnecessary restart.
This could potentially be avoided by checking the value of force_restart
when an update call is received, but I prefer to avoid complicating the
logic.