More shutdown improvements

Builds off of https://github.com/LearnBoost/kue/pull/321

This pull request includes additional improvements to the shutdown functions.  These have all been put into place as part of my work to get variable redis key prefix support running.

Summary of changes:

* Redis module/client is properly reset on kue.shutdown
* Worker shutdown is somewhat simplified and should be much faster (removes what looks to be an extraneous setTimeout).