Implemented ban lurker friendly bans

This change will prevent the ban list from growing big and slowing down Varnish.
Info about lurker-friendly bans: https://www.varnish-cache.org/docs/3.0/tutorial/purging.html#bans

I tested this improvement on a very big shop and and several average sized shops.
After this change the ban list (total active bans) is kept clean on the average shops. On the big shop the ban list is a lot smaller than before. This means performance improvement, because each hit is checked against the whole ban list.

Make sure the ban-lurker is enabled by setting the `ban_lurker_sleep` to non-zero.
Default it is set to 0.01 since Varnish 3.0. You can check with: `varnishadm 'param.show ban_lurker_sleep'`
To get rid of old non-lurker-friendly bans after installing this update, you need to restart Varnish.

Kind regards,
Jeroen Vermeulen - Freelance Mageno, PHP & Hosting expert - info@jeroenvermeulen.eu