Improved page prefetching

* Implement proper priority-based page fetching so that more important
  data (coarser level of detail; closer to camera) comes in first.

* Fetch pages based on tree traversal from previous frames (will result
  in an empty first frame), respecting priority.

* Change prefetch priority ordering so that higher numbers result in
  higher fetch priorities.  This seems more intuitive than the default
  sort ordering based on <.