Better monitoring for webserver response times

In the wake of #40566 (closed), it was shown that our monitoring infrastructure isn't sufficiently sensitive with respect to web server response times. We had an ongoing DoS on the static mirror hosts for days and we only noticed when the response times consistently surpassed 10 seconds.

We should probably modify the existing checks or add new ones that will monitor whether the static mirror host (or even any web host) is serving pages within an acceptable delay, say 1 second.