Policy - add destructible interface

Summary: Fixes T6957. If / when a policy object is destroyed, access to an object that uses that policy object is denied.

Test Plan: looked around in the code to fail confident enough to write the summary above

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6957

Differential Revision: https://secure.phabricator.com/D11380