Geo migration automatic DDL transactions are against the wrong database

The following discussion from !4719 (merged) should be addressed:

• @nick.thomas commented on a discussion: (+4 comments)

  Argh, I suspect what's happening here is that the automatic DDL transaction is against ActiveRecord::Base rather than Geo::TrackingBase

  Let's leave this MR as-is, but we should have a ~"technical debt" issue to resolve this as it's very surprising.

I suspect this is what's happening, anyway.

By default, a migration is wrapped in a transaction. When performing SQL that needs to be run inside a transaction against the tracking database, we noted that the SQL complained it was not in a transaction.

Best theory: the automatic transaction is against ActiveRecord::Base rather than Geo::TrackingBase, and so completely the wrong database.

We should fix this so that the automatic transactions can successfully roll back partially applied migrations on the tracking DB.