Don't bother with Ember.run.once, as 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
another quick fix, which should reduce run-loop pressure.
Merge pull request #1544 from stefanpenner/be-nice-to-the-run-loop

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