light journal refactoring and comment improvements
light journal refactoring and comment improvements

Summary:
Before I make further changes to the Journal, improve the comments and
refactor a few small things.

Reviewed By: kmancini

Differential Revision: D24089530

fbshipit-source-id: de9da2c1e6b1c87b6587781cfa55ae7cc4085eeb

nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman