proxy_cache_background_update leads to 200 ms delay

[stephan13360]

To clarify a few things:

The delay has nothing to do with the proxy_cache_valid 200 time, this change
only made me realize that there seems to be a problem. Before this change
our web monitoring would always get a cache HIT, and after it mostly got a
STALE response because it checks every minute.

We use NGINX 1.15.0 mainline.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,280434,280437#msg-280437