Disable re-reading of config file

Since the objects read from the config file are passed to the various
threads, it's unsafe to re-read the config file (and throw away
ConfigWriter._config_data). As such, we disable the re-reading of the
file (since now the master is the owner the file, it makes not sense to
re-read it), and any modifications to the file must be done offline,
otherwise they will be overwritten.

Reviewed-by: imsnah