Decrease memory usage of updater

This release decreases memory usage of updater during update process
about 40%. Memory usage is still not ideal but this improves stuff a
lot. There is disadvantage to this change as it increases storage
requirements during update from at maximum twice size of all packages
(installed version and not yet installed one) to thee times (+download
packed files).

* pkgupdate now downloads packages to /usr/share/updater/download
  instead of ram.
* pkgupdate now downloads packages only when plan is approved, not
* Added memory usage test bench