Promote the poll mutex to be a session mutex so that we can use it

Promote the poll mutex to be a session mutex so that we can use it
for the VCL reference as well, this saves locking operations.

Call the atual VCL code when we get the request.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@85 d4fa192b-c00b-0410-8231-f00ffab90ce4