Comet fixes

This fixes a handful of bugs that when taken together kept things mostly working!

  - Unify lastListenerTime (which was previously always zero) with
    lastListenTime
  - Make lastRenderTime, delta timestamps, and lastListenerTime
    consistent with each other as they are directly compared
  - More accurately track whether any deltas have been sent
  - Pretend initial render happened at creation time to prevent a second
    render call from running on the initial Listen message
  - Rename devMode to alwaysReRenderOnPageLoad
(Worth noting: I'd be down to do this refactoring myself, since I've also been meaning to make the two anonymous comet subclasses in `LiftSession` named subclasses for clarity.)