Don't bother with Ember.run.once, if we can detect an impending flush

by inspecting the size of the local queue of _deferredTriggers

This should mitigate some costly run-loop pressure.

Inspired to help with: https://github.com/ebryn/backburner.js/issues/51


This can likely be wrapped up in a nice helper (or a pushed down into backburner as a feature of some sort)