R1:a3253f78ce14

Make query engines "overheat" instead of stalling when filtering too many results

Summary: Ref T11773. This is an initial first step toward a more complete solution, but should make the worst case much less bad: prior to this change, the worst case was "30 second exeuction timeout". After this patch, the worst case is "no results + explanatory message", which is strictly better.

Test Plan:
Made all feed stories fail policy checks, loaded home page.

- Before adding overheating: 9,600 queries / 20…