Add Varnish 4.0.x compatibility to the varnish plugin

Many of the fields have changed in `varnishstat` 4.0.x

Additionally, the `type` element is present for all stat entries. Thus to distinguish between 'grouped' stats we need to use the `ident` element instead.

This PR fixes all of the non-DEBUG meters such as hit_rate, transfer_rates etc. so they display correctly for Varnish 4.0.x. It should still remain compatible with Varnish 3.x since I merely added the new stat entries and left the old ones there. The plugin is smart enough to ignore stat entries it cannot find. :-)