Warn on long event loop delay

[mrocklin]

We emit a warning whenever the event loop is delayed by more than one
second.

[mrocklin]

cc @kuanb @adamklein do we want to do something like this? This would make your problems more evident, but also somewhat loud in the logs

[adamklein]

I'll add it in and see what kind of verbosity I see.

[adamklein]

I like this. It's not too bad at all. I definitely get warnings. I just don't know what causes them... yet

[kuanb]

Sorry I am late to the party on this one - I just wanted to say I second @adamklein. This looks really helpful, esp in helping cue the user into roughly where the issue is emanating from. Thanks.