Add queue package

This is based on the [Ruby QueueClassic library](https://github.com/QueueClassic/queue_classic).

Opening now for early feedback.

Prerequisite for #536 
Yes, on our fork: https://github.com/flynn/go-sql/commit/57345fbaa6a93d53c05dd4fdbaef6b8c807d755a

Do you think we'll be merging that upstream?