Revive the backend_conn counter
Revive the backend_conn counter

Discussed with Federico.

Fixes: #1725

feld/Varnish-Cache,feld/Varnish-Cache,varnish/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,varnish/Varnish-Cache,varnish/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,varnish/Varnish-Cache,varnish/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache