sync with 4.0.0-beta1
upgrade this too for varnish >= 4.0.0-beta1
fix example: client.* are not available in vcl_backend_*

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.

dixit current documentation: https://www.varnish-cache.org/docs/4.0/whats-new/changes.html

Variables client.* was allowed in vcl_backend_* until 4.0.0-beta1 (see commit
dc8b9fc9b309106321f2cdd1793a68f36f5f0287) but assertion (ctx->req) != NULL was triggered from
function VRT_r_client_ip (bin/varnishd/cache/cache_vrt_var.c)