Varnish 2.0 bonus feature #1: backend->max_connections

Varnish 2.0 bonus feature #1:  backend->max_connections

        backend default {
                .host = "127.0.0.1";
                .port = "9080";
                .max_connections = 100;
        }

will limit the simultaneous TCP connections to this backend to 100.

Stats counter backend_busy == "Backend connections too many" tells
how often we have hit this limit for a backend.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@3135 d4fa192b-c00b-0410-8231-f00ffab90ce4