Allows you to turn off esi processing for a specific request using set req.esi = off even if the object has been esi processed -- this is useful if you have a cache heirarchy of varnish machines as well as debugging. Only change to processing is to store a copy of the attribute value instead of mangling to original. Committed from Oahu.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4066 d4fa192b-c00b-0410-8231-f00ffab90ce4