Serve ESI-includes from a different backend.

Serve ESI-includes from a different backend so that we don't race
against when the backend connection is ready for reuse.

Fixes: #1440

bin/varnishtest/tests/e00019.vtc

@@ -22,13 +22,25 @@ server s1 {
 	chunked {<esi:comment/>}
 
 	chunkedlen 0
+} -start
 
+# The included object gets served from a different backend.
+# This is to avoid a race between when a backend connection
+# gets put up for reuse because of background fetches in
+# Varnish 4
+server s2 {
 	rxreq
 	expect req.url == "bar/foo"
 	txresp -body {<INCL>}
 } -start
 
 varnish v1 -vcl+backend {
+	sub vcl_backend_fetch {
+		if (bereq.url != "bar") {
+			set bereq.backend = s2;
+		}
+	}
+
 	sub vcl_backend_response {
 		if (bereq.url == "bar") {
 			set beresp.do_esi = true;