Clean up expired messages for SQLAlchemy driver

Change-Id: I1d14258528bdea89399d491d45e5e1dfd1b06b50
Closes-Bug: #1331228
Don't call config() in the global space

Calling `config` in the global space breaks the config generator.
There's another case like this in the wsgi transport but it's not loaded
as part of the config generation step.

Change-Id: If4bccf54e89994c665bb1d5f4f0766e7b7f16c6d
Merge "Clean up expired messages for SQLAlchemy driver"
Merge "Don't call config() in the global space"