Update subresource prediction schema

- Rename 'hits' to 'loads' for the top-level resource
  This makes things make more sense, since it's not really a "hit"
- Add last_load time for top-level resource
- Add last_hit time for subresource
  These let us be smarter about decaying what actions we'll take for
  particular subresources (can avoid doing preconnect for things we
  didn't hit last time, can decay *all* actions for pages we haven't
  loaded in a long time)
More schema updates
Even more schema updates