Significantly reduced complexity in rocket.py - No more monkeypatching needed.
- predownload hook is now used to download everything in parallel prior to the
  typical getPackages() calls. This allows us to do the download in parallel
  and still defer all postprocessing to yum itself, since it will check if a
  cached copy of each package exists before trying to download it again.
- No more duplicated logic, or calling plugins from other plugins.