emit a warning if the event loop delay is over a user-determined threshold.

[mcollina]

Accumulated event loop delay is great if we need it for stats purposes, however it will always grow to infinity. In long-running processes, something that is more valuable is to know if the event loop delay grows over a given threshold, plus the latest, point-in-time delay.

This enables 503 protection like https://github.com/mcollina/loopbench/blob/master/example.js#L20-L23.