Purging eligible entities

After the purge graph has been built and all nodes have been marked as needed, PolicyCenter purges eligible entities as follows:

1. Purge all purgeable nodes and associated non-root entities.
2. When purging a policy, if the AccountContactRole entity and the AccountContact entity on the account associated with the policy have been orphaned, they are purged also.

   Contacts whose only link to the graph is the policy being purged are unlinked from the graph in this step. If the contact was marked not purgeable as a result of propagation of the Do Not Destroy flag, the contact becomes purgeable again. If the contact had a Do Not Destroy flag that was directly set, that flag is left alone and the contact remains not purgeable.

3. Purge any contacts that no longer have links with the graph and have a disposition other than MUST_NOT_DESTROY.