Rewrite the req.hash implmentation:

Rewrite the req.hash implmentation:

Instead of assembling the entire hash-string in the workspace, use
a scatter gather approach, hinted by the VCL compiler.

This eliminates the workspace reservation which prevented regsub() from
working in vcl_hash, and reduces the size of the necessary workspace a
fair bit as well, at the cost of a little bit of complexity in the
hash implmentations.

Closes ticket 137 and possibly 141

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1805 d4fa192b-c00b-0410-8231-f00ffab90ce4