Switch from Winston to Log.js for logging.

This mainly stems from Error objects, as well as other types not always
being logged to winston if passed as an argument:

https://github.com/flatiron/winston/issues/280
https://github.com/flatiron/winston/issues/177

We're not really using some of Winston's more advanced features right
now, so log.js seems to do the trick.
Don't cache null values in redis for dns.