add hot_config_file config parameter

add hot_config_file config parameter

This allows changing certain variables without restarting the
master process or code reload.  Currently, only the following
variables are supported: @timeout, @nr_workers, @hot_config_file.

Any other config changes will/should require re-executing the
running binary.  This config file is run through eval(); so it
really users plenty of rope to hang themselves with.  Of course,
it requires valid Ruby syntax:

------------------------- 8< ------------------------
@nr_workers = 8
@timeout = 15
@hot_config_file = "/var/tmp/new_hot_config_file"
------------------------- 8< ------------------------

Lowering the timeout will trigger all existing workers to be
gracefully stopped and restarted.

This file is loaded at startup, and overrides any config
settings that may already be loaded.