Support ESI with clustering

Previously, we failed to support ESI correctly: The fetch node would
run ESI on the client side, such that the delivery node would receive
an already ESI processed object.

The concept of this change is:

Custom code on the fetch node decides on beresp.do_esi. vcl_deliver on
the fetch node sets the "E" bit of a new flags header if obj.can_esi
is true, which reflects beresp.do_esi and disables ESI processing.

When the delivery node receives an object from the fetch nodes, it
sets beresp.do_esi based on the flags header.

Ultimately, beresp.do_esi == obj.can_esi is identical on the fetch and
delivery nodes.

pkg/varnish/vcl/self_shard_tmpl.go

@@ -209,6 +209,9 @@ sub vcl_backend_response {
                 else {
 			set beresp.uncacheable = true;
 		}
+		if (beresp.http.VK8S-Cluster-Flags ~ "^E") {
+			set beresp.do_esi = true;
+		}
 		return (deliver);
 	}
 }
@@ -221,12 +224,17 @@ sub vcl_backend_error {
 
 sub vcl_deliver {
 	unset resp.http.VK8S-Cluster-TTL;
+	unset resp.http.VK8S-Cluster-Flags;
 	if (remote.ip ~ vk8s_cluster_acl
 		{{- if hasPrimary . }} && ! vk8s_cluster_primary.defined()
 		{{- end }}) {
 		if (! obj.uncacheable) {
 			set resp.http.VK8S-Cluster-TTL = obj.ttl;
 		}
+		if (obj.can_esi) {
+			set resp.http.VK8S-Cluster-Flags = "E";
+		}
+		set resp.do_esi = false;
 		return (deliver);
 	}
 }