configfile: convert to es6 class

[msimerson]

• configfile: convert to es6 class
• configfile.read_dir: promisify
• configfile: use simpler es6 for..in and for..of
• configfile.watch_dir: move declared function to callback (for this scoping)

[msimerson]

Note for reviewers, the first commit (convert to es6 class) has a ton of noise and no real functional changes. It's much easier to review the following 3 commits individually, with the ignore whitespace option enabled.