General logging cleanups.

You should be able to run rejester with INFO level logging on and get
a pleasant flow of messages as legitimately interesting things happen.
In 'rejester run_workers', make --logpath and --pidfile truly optional,
and only set up logging after we get into the worker's world.  (This
means that you can specify a fixed FileLogger in a config YAML file
and log messages from all workers will end up there.)