Turns out when Windows is running in QEMU and a window scroll happens,
there are lots of little hextile rects sent. This is slow in noVNC.

- Some recording/playback improvement.
- Add test harness to drive playback of recordings.
- By pulling off the rect header in one chunk we get a 3X speedup in
  Chrome and a 20% speedup in firefox (specifically for the scroll
  test).
- Also, get rid of some noise from creating timers for handle_message.
  Check to make sure there isn't already a pending timer first.