Fix PG::ConnectionBad issue

[ukd1]

Checking out the fixes @siegy22 did related to #302.

Let's lazy use the ActiveRecord::Base.connection.raw_connection.

[bwlang]

This is working in my hands (had the problem before the patch, working now)

gem 'rails', git: 'https://github.com/rails/rails.git', branch: '6-0-stable'
gem 'queue_classic', github: 'siegy22/queue_classic', branch: 'master'

[siegy22]

The tests need to be adjusted tho, didn't do that yet. And it's a breaking API change of the ConnAdapter, so this should definitely land in a major release 😄

[siegy22]

And thanks for your effort @ukd1, I'm here if you need some help.