[1.2.X] Fixed #14700 - speed up RawQuerySet iterator.

This moves constant work out of the loop, and uses the much faster *args
based model instantiation where possible, to produce very large speed ups.

Thanks to akaariai for the report and patch.

Backport of [14692] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14693 bcc190cf-cafb-0310-a4f2-bffc1f526a37