switch loggers from using FileHandler to WatchedFileHandler

- This change makes a seamless switch to using WatchedFileHandler
instead of FileHandler for the main logger and the builtin plugins.
If a system log rotation utility moves the circus log, circus will
continue to function fine when logging.

The only downside is a slight performance hit as WatchedFileHandler
stats the log file on each log call, but it seems like a good tradeoff
for a systems tool like circus to help ensure that the circus logs never
cause problems.