Create mechanism to collapse migrations betwen major versions

[ejether]

We don't want to carry around every migration ever and we may want to upgrade/improve the migration class in a non-backwards-compatible way.

I suggest we remove all migrations from the previous major version and only carry migrations for the current major version. There will need to be some better logic for 'edge sensing' where, for example, a 2.9 -> 3.0 migration.