[iOS] Fix some CoreData issues

Description

One of many different issues: https://fabric.io/edx-inc/ios/apps/org.edx.mobile/issues/573219efffcdc04250a11613

Acceptance/Test Criteria:
This is a big change in the core data handling, and so we need to make sure video downloads, playback, and last accessed courseware sections are not affected.

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Status

Assignee

Christopher Lee

Reporter

Michael Katz

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