debug: limit size of json debug messages

JSON can be very large, especially with CPU and Heap snapshots.

/to @rmg 

I am considering making a strong-debug module, this identical boiler-plate `lib/debug.js` goes in most of my projects (though `debug.json()` is only shared with strong-supervisor, ATM). I'm also thinking that is a small, sad, npm package, though. What to do, suggestions?