Changes in Varnish 4

Varnish 4 is quite an extensive update to Varnish 3, with some very big improvements to central parts of varnish.

Client/backend split

In the past, Varnish has fetched the content from the backend in the same thread as the client request.In Varnish 4 we have split the client and backend code into separate trheads allowing for some much requested improvements. This split allows Varnish to refresh content in the background while serving stale content quickly to the client.

This split has also necessitated a change of the VCL-functions, in particular functionality has moved from the old vcl_fetch method to the two new methods vcl_backend_fetch and vcl_backend_response.