Revert "Load rows even if importlimit = 0"

This reverts commit a579ffa7dd39567f259b7da0ebda16fcc804620a.
Revert "Make db_session more manageable"

This reverts commit 282e84282c7abbb4ca5ed918fab439cbbe71a9cb.
Revert "Make the import process more managable"

This reverts commit d576da3b08f486a9785ffc448d5ffbfe848e6ea5.
Reduce the memory footprint with the multiprocessing module

Python likes to acquire memory but not release it back to the operating
system until the process shuts down.

Using SQLAlchemys ORM system requires prefetching rows at various
points, making the memory usage go up.

Using a new process via the multiprocessing module makes sure the main
process doesn't allocate ever more memory until the system grinds to a
halt.

More background can be found at:

* http://blip.tv/pycon-us-videos-2009-2010-2011/pycon-2011-how-dropbox-did-it-and-how-python-helped-4896698 (from minute 23)
* https://stackoverflow.com/questions/1145905/sqlalchemy-scan-huge-tables-using-orm?rq=1
* https://groups.google.com/forum/#!topic/sqlalchemy/yAlrzMAH-Yk