When streaming, send headers if no body data available yet

This is a TTFB optimization: I noticed that, when streaming, we would
only sometimes send headers while waiting for more body data from the
backend, depending on the amount of body data read ahead when reading
the backend headers.

The flush_head debug bit would avoid the delay, but it leads to an extra
flush in all cases, even when we could send some body data down the line
with the headers.

This trivial patch peeks into the busy object to determine if the
first ObjWaitExtend() is likely to block and, if so, issues a flush,
which will cause the headers to be sent in addition to any initial data
VDPs might have to send.

bin/varnishd/storage/storage_simple.c

@@ -280,6 +280,8 @@ sml_iterator(struct worker *wrk, struct objcore *oc,
 	l = 0;
 
 	u = 0;
+	if (boc->len_so_far == 0)
+		func(priv, OBJ_ITER_FLUSH, NULL, 0);
 	while (1) {
 		ol = len;
 		nl = ObjWaitExtend(wrk, oc, ol);