Create a more-robust upgrade routine

[alexstandiford]

Batch tasks are a great start, but really, an upgrade routine should be able to:

1. Upgrade something
2. Revert something if it was somehow botched midway
3. Downgrade something to a previous version

This would make it possible for a plugin to better-repair itself should it somehow not go as-expected, and also make it possible to "put something back" if the user decides that a downgrade is necessary.