refactor: handle duplicates while using the enforcer directly

[mariajgrimaldi]

Description

While loading policies from a datastore to another using the enforcer directly, we should make the operations on the enforcer with the latest policies. To do this, we should reload the policy each time a write operation was performed to ensure we're using the latest policies in the datastore.

How To Test

Load the policy in the LMS / CMS service by running ./manage.py lms load_policies and see how each load ends with the same results:

Screencast.from.15-10-25.18.05.30.webm

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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