LavinMQ::PolicyTarget

This can be overriden if the child class needs to do something after a policy has been applied.

Policy arguments are staged by clear_policy_arguments and apply_policy_argument, and only take effect in commit_policy_arguments, so a re-apply never exposes the resource without its policy.

A generation older than the newest one applied means a later apply_policies run, with newer policies, got here first: skip it.

Return true if applied