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
Aha, gotcha!

So I'm wondering if we can refactor so that we have a trait that provides the basic comet interaction + partial updating, then layer a trait on top that brings us to parity with `CometActor`, namely by adding the render-related stuff in there. It seems like rendering is layered on top of partial updating, rather than the other way around, including things like clearing the partial updates when render is called and all that.

The other benefit we get from that is that things like the anonymous actor that handles streaming promises can use only the partial updating trait, since it never uses the rendering aspect.

What do you think?