Tests single type shutdown calls used by pause
Fixes worker shutdown issues

Using `this` in `_fn` is a bug.  `clients` was never getting cleaned up.

This change also removes all listeners from the worker object so there aren't
any weird events firing after shutdown has happened.
Nulls out exports.workers

exports also needs to be cleared on shutdown.  Otherwise, the exports will just
stack up with all previous workers in tests.
Shorter shutdown timer, comments
Removes worker from Kue on single type shutdown
Fixes issue where this test failed on fresh redis state