Workers than can work multiple queues.

Here's my attempt at #169

A couple of discussion points:

* The semantic of a _queue_ becomes a bit icky. Is it a single named queue, or is it a queue of multiple 'lanes' from a worker's perspective?
* Could you please review my `conn.execute` statements, is there a better way to pass the arrays in?
* The multi-queue work order is determined with the string position, and will fail with similar queue names - is there a better way?