Add a new paramter "purge_hash" which defaults to "off".

Add a new paramter "purge_hash" which defaults to "off".

Only save the hash-string in the session workspace and objects when
this paramter is set to "on".

For sites with many small objects, this will save significant VM.

When this paramter is set to "off", the "purge.hash" facility will
not work, but this should not be a problem, because the new purging
facility allow much more expressive purging, the typical case
being:

	purge req.http.host ~ www.foo.com && req.url ~ "article2383"

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@3526 d4fa192b-c00b-0410-8231-f00ffab90ce4