The watchdog mechanism currently triggers when any queueing is happening,
regardless of the priority. Strictly speaking it is only the backend
fetches that are critical to get executed, and this prevents the thread
limits to be used as limits on the amount of work the Varnish instance
should handle.

This can be especially important for instances with H/2 enabled, as these
connections will be holding threads for extended periods of time, possibly
triggering the watchdog in benign situations.

This patch limits the watchdog to only trigger for no queue development
on the highest priority queue.