Refactor to allow updates.

The work to [integrate with islandora_batch](https://github.com/Islandora/islandora_importer/pull/27) broke the manner in which we were performing updates, so here we rework some things to make it work again.

Dependent on https://github.com/Islandora/islandora_importer/pull/46
The reference here is heavy wizardry that is likely unnecessary.