update back burner

Diff: https://github.com/ebryn/backburner.js/compare/919861fa9f3a5504c37f05607b1b02421af04167...baef3625dd4374e3803288652cd0aab68775cc62

* [Cleanup]unify deferredActionQueue.flush and queue.flush
* [Perf] reduce cost of flushing and dramatically reduce cost of flushing empty queues.