The primary motivation behind this feature was to expose a path
for dealing with multiple database connections within a single process.
Specifically, I wanted to test the worker's ability to block on a LISTEN
and subsequently wait when another connection had issued a NOTIFY. This
was not possible with our global Conn module.

This change has introduced a new abstraction known as the ConnAdapter.
The ConnAdapter is explicity set on the Queue. The adapter simply wraps
a few commands around an instance of a PG::Connection. The adapter was also
designed so that clients can pass their active record connections to the
wraper for cases when you want to share a connection between queue_classic
and a rails app.

Even if the worker will be responsible for many queues, it should
not maintain a seperate database connection for each queue.

This change also provides a correct test for the worker's ability to wait.
Previous test was completely wrong.

cc: @collin