explicitly disable the send deadline on the backend side

As @dridi and myself concluded, the send_timeout had no effect on
backend connections anyway because we never set SO_SNDTIMEO (aka
idle_send_timeout on the client side) on backend connections.

With the next commit, we will fix the send_timeout on the client side and
thus would also enable it for "dripping" writes on the backend side.

To preserve existing behavior for the time being, we explicitly disable
the timeout (actually deadline) on the backend side. There is ongoing
work in progress to rework all of our timeouts for 7.x.

Implementation note: if (VTIM_real() > v1l->deadline) evaluates to false
for v1l->deadline == NaN

Ref varnishcache#3189

bin/varnishd/http1/cache_http1_fetch.c

@@ -97,9 +97,8 @@ V1F_SendReq(struct worker *wrk, struct busyobj *bo, uint64_t *ctr_hdrbytes,
 	}
 
 	VTCP_blocking(*htc->rfd);	/* XXX: we should timeout instead */
-	/* XXX: what is the right timeout ? Isn't send_timeout client side? */
-	V1L_Open(wrk, wrk->aws, htc->rfd, bo->vsl,
-	    bo->t_prev + cache_param->send_timeout, 0);
+	/* XXX: need a send_timeout for the backend side */
+	V1L_Open(wrk, wrk->aws, htc->rfd, bo->vsl, nan(""), 0);
 	hdrbytes = HTTP1_Write(wrk, hp, HTTP1_Req);
 
 	/* Deal with any message-body the request might (still) have */